The prosecutor in the case of James Holmes, suspected in the shooting deaths of 12 moviegoers in Colorado, said Monday that the prosecution has an "enormous amount of evidence," but that she would not call it a "slam dunk."

"There is no such thing as a slam dunk case ... we would never presume that it would be a slam dunk. We will work very hard on this case to prosecute it," Arapahoe County District Attorney Carol Chambers said.

Holmes -- who faces the possibility of life in prison or the death penalty for allegedly carrying out one of the most deadly mass murders in U.S. history -- might have to plead insanity, according to veteran Seattle attorney and legal analyst Anne Bremner.

"The insanity defense appears to be the only option," Bremner told The Huffington Post. "We don't hold those who don't have the requisite criminal intent criminally accountable."

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"Competent psychiatrists will most likely first evaluate his competency to stand trial," Bremner added. "We don't know if he has any mental illness as yet. Many of us assume he does or he wouldn't have committed his horrific acts but he could just be bad -- a natural-born killer -- and competency is a very low standard."

Holmes, a 24-year-old former doctoral student at the University of Colorado, Denver, has been held on first-degree murder charges in the shooting spree at a midnight screening of "The Dark Knight Rises."

Police arrested Holmes in the early morning hours of July 20, minutes after the shooting in the sold-out theater in Aurora, Colo. Holmes, dressed in ballistic gear and armed with an assault rifle and three other guns, set off gas canisters before opening fire, police said. A dozen theater patrons were killed and 58 others were wounded.

Holmes will face formal charges from prosecutors on July 30 and Chambers said her office is considering the death penalty against him.

The prosecution could, however, face a challenge in aiming for the death penalty. There is always the possibility that the defense could claim insanity, which, if successful, would make Holmes ineligible for the death penalty.

"Maybe his best defense is to argue at sentencing you can't execute the mentally (if he is) ill. That it is cruel and unusual punishment under the Eighth Amendment," Bremner said.

PHOTOS: (Story Continues Below)

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  • James E. Holmes

    James E. Holmes, left, appears in Arapahoe County District Court, with defense attorney Tamara Brady, right, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting on Friday, July 20, in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes, Tamara Brady

    People in the courtroom listen to the court hearing of James E. Holmes in Arapahoe County District Court Monday, July 23, 2012, in Centennial, Colo. Holmes is accused of killing 12 and wounding 58 in a shooting rampage in a Colorado theater on Friday, July 20. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    James E. Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • In this image taken from video provided by KUSA.com, James Holmes, left, the suspected gunman in Friday's Colorado theater massacre, makes his first appearance in court with his attorney Tamara Brady in Centennial, Colo. on Monday, July 23, 2012. (AP Photo/KUSA.com)

  • Carol Chambers

    Carol Chambers, district attorney for the 18th Judicial District, speaks during a media availability after a court appearance by James Holmes at the Arapahoe County Courthouse, Monday, July 23, 2012, in Centennial, Colo. Twelve people were killed and dozens were injured in a shooting attack early Friday at a packed theater during a showing of the Batman movie, "The Dark Knight Rises." Police have identified the suspected shooter as Holmes, 24. (AP Photo/Alex Brandon)

  • James E. Holmes, Tamara Brady

    James E. Holmes, center, appears in Arapahoe County District Court, with public defenders Tamara Brady, right, and Daniel King Monday, July 23, 2012, in Centennial, Colo. Holmes is accused of killing 12 and wounding 58 in a shooting rampage in a movie theater on Friday, July 20 in Aurora, Colo. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    James E. Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting Friday, July 20, in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • David Sanchez

    David Sanchez, whose son-in-law was critically wounded, speaks after a court appearance by shooting suspect James Holmes at the Arapahoe County Courthouse, Monday, July 23, 2012, in Centennial, Colo. Twelve people were killed and dozens were injured in the shooting attack early Friday at an Aurora, Colo., theater during a showing of the Batman movie, "The Dark Knight Rises." (AP Photo/Alex Brandon)

  • Family members of a victim from last week's theater shooting in Aurora, Colo., link hands as they arrive for a court appearance by shooting suspect James Holmes at the Arapahoe County Courthouse, Monday, July 23, 2012, in Centennial, Colo. Twelve people were killed and dozens were injured in the shooting attack early Friday at the packed theater during a showing of the Batman movie, "The Dark Knight Rises." (AP Photo/Alex Brandon)

  • A woman, third from left, is led to the courtroom where James Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is the suspect in the mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Barry Gutierrez)

  • Anggiat Situmeang,

    Anggiat Situmeang, center, who suffered an injury in last week's theater shooting in Aurora, Colo., is helped inside to see a court appearance by alleged shooter James Holmes, at the Arapahoe County Courthouse Monday, July 23, 2012, in Centennial, Colo. (AP Photo/Alex Brandon)

  • James E. Holmes

    Assistant District attorney Karen Pearson speaks at a hearing for James E. Holmes in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    James E. Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    James E. Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    James E. Holmes, left, appears in Arapahoe County District Court, with defense attorney Tamara Brady, right, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting on Friday, July 20, in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

  • James E. Holmes

    District Court Judge William Blair Sylvester speaks as James E. Holmes appears in Arapahoe County District Court, Monday, July 23, 2012, in Centennial, Colo. Holmes is being held on suspicion of first-degree murder, and could also face additional counts of aggravated assault and weapons violations stemming from a mass shooting last Friday in a movie theater in Aurora, Colo., that killed 12 and injured dozens of others. (AP Photo/Denver Post, RJ Sangosti, Pool)

Public defender Tamara Brady was assigned to the case, presumably after Holmes indicated he could not afford to hire a lawyer. Brady appeared in court with Holmes on Monday for an initial hearing. Her client seemed sleepy or dazed and often had his eyes shut during the proceeding. Brady has yet to comment on her client’s behavior in court and the defense has thus far been reluctant to speak on the case.

"He looks insane, his acts are those of an insane person, ergo he is insane," Bremner said.

The veteran attorney cautions that there is a long road ahead for the defense.

"His defense will not be that easy," she said. "In fact, his attorneys face nearly insurmountable difficulties in a court of law and even more so in the court of public opinion. No one even wants to talk about James Holmes, let alone acknowledge that he has a right to a defense and a trial."

Bremner said Holmes' attorney may well be an "attorney for the damned," due to the number of things she would need to overcome at trial. The biggest challenge for the defense may be the allegation by police that Holmes spent months planning and stockpiling guns, thousands of rounds of ammunition and head-to-toe ballistic gear.

"He planned, he schemed [and] he purchased. He stockpiled, he booby-trapped, he created mass mayhem and death. He shot a six-year-old who just learned how to swim. He took 12 souls and shattered the lives of hundreds more. He showed no remorse. He remained silent and he wouldn't cooperate with the authorities," Bremner said.

"When I was a DA, we would say, 'well he didn't hold up a telephone pole with a banana,'" she said. "He robbed a bank with a gun."

Earlier on HuffPost:

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  • Tom Sullivan , holds a photograph of his son, Alex Sullivan, as he pleads with the media to help him find his son, outside Gateway High School on Friday, July 20, 2012 in Aurora, Colo. Alex Sullivan, was celebrating his 27th birthday by attending midnight premiere of the Batman movie Friday night. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A woman cries outside Gateway High School where witness were brought for questioning after a gunman opened fire at a midnight premiere of The Dark Knight Rises Batman movie Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Tom Sullivan, center, embraces family members outside Gateway High School where he has been searching frantically for his son Alex Sullivan who celebrated his 27th birthday by going to see "The Dark Knight Rises," movie where a gunman opened fire Friday, July 20, 2012, in Aurora, Colo. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Chandler Brannon, 25, sits outside Gateway High School where witnesses were brought for questioning after a shooting at a movie theater showing the Batman movie "The Dark Knight Rises," Friday, July 20, 2012 in Aurora. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A small group prays outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Family and friends wait outside Gateway High School where witnesses were brought for questioning after a shooting at a movie theater showing the Batman movie "The Dark Knight Rises," Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Jacob Stevens, 18, hugs his mother Tammi Stevens after being interview by police outside Gateway High School where witnesses were brought for questioning Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Isaiah Bow hugs his mother Shamecca Davis after being questioned by police outside Gateway High School where witnesses were brought in, Friday, July 20, 2012 in Aurora, Colo. After leaving the theater Bow went back in to find his girlfriend. "I didn't want to leave her in there. But she's ok now," Bow said. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Emma Goos, 19, hugs her mother, Judy Goos, outside Gateway High School where witnesses were brought for questioning Friday, July 20, 2012, in Aurora, Colo. Emma was in the third row of the theater of the new Batman movie when the shooter entered. She helped apply pressure to a man's head who was injured. (Photo credit: AP Photo/Barry Gutierrez)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 50 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNER JONATHAN CASTNER/AFP/GettyImages)

  • Police tape cordons off the parking area around the Century 16 movie theater in Aurora, Colorado, July 20, 2012 where a gunman opened fire during the showing of the new Batman movie. At least 12 people were killed and around 50 wounded in the cinema shooting. (Photo credit: AFP PHOTO JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 40 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 50 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars are seen in the parking area around the Century 16 movie theater in Aurora, Colorado, July 20, 2012 where a gunman opened fire during the showing of the new Batman movie. At least 12 people were killed and around 50 wounded in the cinema shooting. (Photo credit: AFP PHOTO JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police use a video camera to look inside an apartment where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Police break out a window of an apartment where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Shamecca Davis hugs her son Isaiah Bow, who was an eye witness to the shooting, outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Denver. After leaving the theater Bow went back in to find his girlfriend. " I didn't want to leave her in there. But she's ok now," Bow said. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Judy Goos, second from left, hugs her daughters friend, Isaiah Bow, 20, while eye witnesses Emma Goos, 19, left, and Terrell Wallin, 20, right, gather outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Denver. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A SWAT team officer stands watch near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 14 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Police gather near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • A SWAT team officer stands watch near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Jacob Stevens, 18, hugs his mother Tammi Stevens after being interview by police outside Gateway High School where witness were brought for questioning after a shooting at a movie theater, Friday, July 20, 2012 in Denver. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

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  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Aurora Police Chief Daniel Oates talks to media at Aurora Mall where as many as 12 people were killed and many injured at a shooting at the Century 16 movie theatre in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Aurora Police Chief Daniel Oates talks to media at the Aurora Mall where as many as 14 people were killed and many injured at a shooting at the Century 16 movie theatre in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • People gather outside the Century 16 movie theatre in Aurora, Colo., at the scene of a mass shooting early Friday morning, July 20, 2012. Police Chief Dan Oates says 12 people are dead following the shooting at the suburban Denver movie theater. He says 50 others were injured when gunfire erupted early Friday at the Aurora theater. Oates says a gunman appeared at the front of one of the Century 16 theaters. <em>Photo Credit: Karl Gehring, <a href="http://www.huffingtonpost.com/news/exposure/" target="_hplink">Denver Post</a>. </em> / AP

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