SANFORD, Fla. -- Prosecutors are winding down their case in George Zimmerman's murder trial after presenting forensics evidence and testimony aimed at refuting Zimmerman's claim he was acting in self-defense when he fatally shot Trayvon Martin.

It is still to be seen if they will call one or both of Martin's parents to the witness stand to testify about whose voice is on 911 recordings of a fight between Zimmerman and Martin that preceded the shooting.

Martin's parents, Sybrina Fulton and Tracy Martin, contend it is their 17-year-old son screaming on the calls, while Zimmerman's father has said it is his son. The screams are crucial pieces of evidence because they could determine who the aggressor was in the confrontation. An FBI expert testified earlier in the week that a person familiar with a voice is in the best position to identify it.

Testimony resumes and prosecutors are expected to rest their case Friday, after jurors return from the Fourth of July holiday.

On Wednesday, prosecutors called forensics experts and college professors who they hoped would weaken Zimmerman's claims of self-defense and professed ignorance of Florida's "stand-your-ground" defense law.

Florida Department of Law Enforcement DNA expert Anthony Gorgone on Wednesday testified that Martin's DNA was not found on the grip of George Zimmerman's gun, and Zimmerman's DNA was not found under the unarmed teen's fingernails. Zimmerman says he shot Martin in the chest to protect himself as Martin reached for his gun during a fight.

While cross-examining Gorgone, defense attorney Don West focused on the packaging of the DNA samples, suggesting they could have led to the samples being degraded. Gorgone told him Martin's two sweatshirts had been packaged in plastic while wet, instead of a paper bag where they can dry out, and when he opened the samples they smelled of ammonia and mold.

Florida Department of Law Enforcement analyst Amy Siewert also testified that tearing and residue on Martin's clothing showed the gun was directly against him when it fired.

The forensic experts' testimony came on the same day prosecutors presented evidence about Zimmerman's work in a college criminal justice course, which they say shows the former neighborhood watch volunteer knew about Florida's self-defense law and had aspirations of becoming a police officer. They also showed he had applied for a police officer job in northern Virginia and applied to go on ride-alongs with Sanford Police officers.

Prosecutors have sought to portray Zimmerman as a vigilante who profiled Martin as the teen walked through the central Florida gated community on a rainy night.

Zimmerman had maintained in an interview with Fox News last year that he did not know about the law. Prosecutors say he did have knowledge of it, however, because the subject was covered in the college class. They called as a witness Alexis Francisco Carter, the military attorney who taught Zimmerman's class that covered the "stand-your-ground" law, which says a person has no duty to retreat and can invoke self-defense in killing someone if it is necessary to prevent death or great bodily harm. Carter described Zimmerman as one of his better students and said the neighborhood watch volunteer got an "A" in his class.

Under cross-examination, Carter gave two definitions of legal concepts that seemed to bolster the defense's case. He explained that a person can make a self-defense argument if the person has a "reasonable apprehension" of death or great bodily harm.

"It's imminent fear. The fact alone that there isn't an injury doesn't necessarily mean that the person didn't have a reasonable apprehension or fear," Carter said. "The fact that there are injuries might support there was reasonable apprehension and fear."

Carter also explained the concept of "imperfect self-defense," when a person is being threatened but then counters with a force disproportionately greater than the force used against them.

"They would have the right to defend themselves?" said defense attorney Don West.

"Right," Carter said.

Another instructor, Seminole County State College professor Scott Pleasants, testified that Zimmerman had taken his online criminal justice class. Pleasants' testimony via Skype from Colorado, broadcast live on television, was interrupted when he started getting inundated with Skype calls.

Zimmerman, 29, is charged with second-degree murder in the shooting death of Martin last year. Martin was black; Zimmerman identifies himself as Hispanic. The case sparked nationwide protests after there was a 44-day delay in Zimmerman's arrest, and it touched off a debate about race and self-defense.

Prosecutors said Zimmerman's ability to understand criminal investigations and desire to be a police officer doesn't show wrongdoing, but is relevant to Zimmerman's state of mind on the night Martin was killed.

"He has applied to be a police officer before, he still wants to be one, according to some of his homework assignments. ... This wasn't some sort of passive thing," said prosecutor Richard Mantei, who noted Zimmerman took a course on how to be a good witness and expressed a desire to go on police ride-alongs. "This is simply a fact the jury ought to know."

When he was interviewed by detectives, Zimmerman spoke "in written police jargon" and talked about "justifiable use of force." He also said he "`unholstered my firearm,' not `I pulled my gun,'" Mantei said.

Defense attorney Mark O'Mara argued the items are irrelevant and asked the judge not to allow them.

"There is no relevance and the suggested relevance will be far more outweighed by the prejudice," O'Mara said.

O'Mara said Tuesday that if prosecutors start bringing up Zimmerman's past, the defense will dig into Martin's past, including fights. The judge had ruled previously that Martin's past fights, drug use and school records couldn't be mentioned in opening statements.

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  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 19, 2012</strong> -- Trayvon Martin, 17, and Tracy, his father, travel from Miami Gardens to Sanford, Fla., to visit the elder Martin's fiancee in her townhome at The Retreat at Twin Lakes. <em>Photo courtesy of <a href="http://www.globalgrind.com" target="_blank">globalgrind.com</a></em>

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 26, 2012</strong> -- Trayvon Martin is walking to the home of his father's fiancee after purchasing items from a 7-Eleven store in Sanford. George Zimmerman, a neighborhood watch volunteer, spots Martin at approximately 7 p.m. and calls police. "We've had some break-ins in my neighborhood, and there's a real suspicious guy," Zimmerman tells police.

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 26, 2012</strong> -- Roughly seven minutes after Zimmerman's call to police, authorities receive a 911 call from an individual reporting a fight. During the call, the dispatcher hears a gunshot in the background and sends police units to the location. Responding officers discover that Martin has been shot in the chest. The teen is unresponsive and pronounced dead at the scene. Police find no identification on Martin and label him a John Doe.

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 26, 2012</strong> -- Questioned by police, Zimmerman informs them that Martin attacked him and he fired his gun in self-defense. Authorities confiscate Zimmerman's 9 mm semi-automatic pistol and take him to the Sanford Police Department for further questioning.

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 27, 2012</strong> -- Following a lengthy interview, George Zimmerman is released from the police station at approximately 1 a.m. Hours later, Tracy Martin contacts police to report his son missing. Investigators soon connect the dots and inform the elder Martin of his son's death. After receiving treatment from a family doctor, Zimmerman meets with investigators and reenacts the events of the shooting at the crime scene.

  • Key Dates In The Trayvon Martin Case

    <strong>March 8, 2012</strong> -- Tracy Martin holds a press conference, during which he criticizes the investigation into his son's slaying. "We feel justice hasn't been served," Martin tells reporters.

  • Key Dates In The Trayvon Martin Case

    <strong>March 9, 2012</strong> -- Martin family attorney Benjamin Crump tells the Miami Herald he is filing a lawsuit for the release of public records in the case.

  • Key Dates In The Trayvon Martin Case

    <strong>March 10, 2012</strong> -- Members of the New Black Panther Party, contending there has been a "miscarriage of justice," rally outside the Sanford Police Department.

  • Key Dates In The Trayvon Martin Case

    <strong>March 12, 2012</strong> -- Sanford Police Chief Bill Lee holds a press conference, at which he claims that investigators were unable to arrest Zimmerman because he was protected by Florida's "Stand Your Ground" law, which allows residents to shoot someone if they reasonably believe they are being threatened. "There is no evidence to dispute Zimmerman's assertion that he shot Martin out of self-defense," Lee says. In response, Martin's parents, Tracy Martin and Sybrina Fulton, post a petition on the Change.org website calling for State Attorney Angela Corey to prosecute Zimmerman. The petition quickly garners support from multiple celebrities and receives nearly 900,000 signatures the first week.

  • Key Dates In The Trayvon Martin Case

    <strong>March 13, 2012</strong> -- In a letter to U.S. Attorney General Eric Holder, the NAACP expresses doubt in the Sanford Police Department's ability to appropriately handle the investigation, asking the Department of Justice to review the case. "The NAACP has no confidence that, absent federal oversight, the Sanford Police Department will devote the necessary degree of care to its investigation," the letter says. Sanford police announce the completion of their investigation and turn the case over to the State Attorney's Office for Brevard and Seminole Counties. "Trayvon Martin and his family, interested persons, and the public-at-large are entitled to no less than a thorough, deliberate and just review of the information provided, along with any other evidence that may or may not be developed in the course of the review process," State Attorney Norm Wolfinger's office says in a statement.

  • Key Dates In The Trayvon Martin Case

    <strong>March 14, 2012</strong> -- Mary Cutcher, a woman listed in police reports as a witness who heard Martin's shooting, <a href="http://www.wftv.com/news/news/witness-sanford-police-blew-us-teen-slaying/nLSqk/" target="_blank">tells WFTV.com that police took only a short statement from her</a> following the shooting. "[The police] blew us off, and I called back again and I said, 'I know this was not self-defense. There was no punching, no hitting going on at the time, no wrestling,'" says Cutcher.

  • Key Dates In The Trayvon Martin Case

    <strong>March 15, 2012</strong> -- Sanford police issue a statement calling Mary Cutcher's TV interviews "inconsistent" with her sworn testimony. Meanwhile, Zimmerman's father, Robert, tells the Orlando Sentinel that his son has been unfairly portrayed as a racist.

  • Key Dates In The Trayvon Martin Case

    <strong>March 16, 2012</strong> -- Sanford police release eight 911 recordings in the case. One of the recordings includes a voice in the background screaming, "Help, help!" The screams are followed by the sound of a gunshot.

  • Key Dates In The Trayvon Martin Case

    <strong>March 19, 2012</strong> -- The Justice Department and the FBI announce they have opened an investigation into the shooting.

  • Key Dates In The Trayvon Martin Case

    <strong>March 20, 2012</strong> -- State Attorney Norm Wolfinger announces that a Seminole County, Fla., grand jury will review the circumstances of Martin's death.

  • Key Dates In The Trayvon Martin Case

    <strong>March 21, 2012</strong> -- The Sanford City Commission votes "no confidence" in Sanford Police Chief Bill Lee and calls for his resignation.

  • Key Dates In The Trayvon Martin Case

    <strong>March 22, 2012</strong> -- Sanford Police Chief Bill Lee holds a press conference and announces he is temporarily stepping down as police chief because his presence is a "distraction." State Attorney Norm Wolfinger recuses himself from the case and Florida Gov. Rick Scott announces that another state attorney, Jacksonville-based Angela Corey, will be replacing Wolfinger as special prosecutor in the investigation. Meanwhile, Rev. Al Sharpton, Martin Luther King III and other civil rights leaders and politicians hold a justice rally at Sanford's Fort Mellon Park. They demand an arrest in Martin's shooting. An estimated 10,000 people attend the event.

  • Key Dates In The Trayvon Martin Case

    <strong>March 23, 2012</strong> -- President Barack Obama tells reporters that the nation needs to do some "soul-searching to figure out how something like this happens." He adds, "If I had a son, he'd look like Trayvon."

  • Key Dates In The Trayvon Martin Case

    <strong>March 24, 2012</strong> -- Members of the New Black Panther Party offer a $10,000 reward for the "capture" of Zimmerman.

  • Key Dates In The Trayvon Martin Case

    <strong>March 25, 2012</strong> -- Rev. Jesse Jackson speaks in Eatonville and encourages revisions to Florida's "Stand Your Ground" law. "If it's a moment, we go home. If it's a movement, we go to war," says Jackson.

  • Key Dates In The Trayvon Martin Case

    <strong>March 26, 2012</strong> -- Police release new details of the investigation, saying Zimmerman told them Martin punched him and slammed his head into the sidewalk several times. Acting Police Chief Darren Scott takes over as chief of the Sanford Police Department. Thousands of people gather in Sanford to mark one month since Martin was killed.

  • Key Dates In The Trayvon Martin Case

    <strong>March 29, 2012</strong> -- Zimmerman's brother, Robert Zimmerman Jr., tells CNN that medical records will prove his brother was attacked and his nose was broken.

  • Key Dates In The Trayvon Martin Case

    <strong>April 3, 2012</strong> -- Florida State Sen. Chris Smith (D-Fort Lauderdale) announces the formation of a task force to review the state's "Stand Your Ground" law.

  • Key Dates In The Trayvon Martin Case

    <strong>April 8, 2012</strong> -- George Zimmerman launches the website "The Real George Zimmerman" to raise money for his defense.

  • Key Dates In The Trayvon Martin Case

    <strong>April 9, 2012</strong> -- State Attorney Angela Corey announces her decision not to use a grand jury in the Martin investigation. The move eliminates the possibility of a first-degree murder charge.

  • Key Dates In The Trayvon Martin Case

    <strong>April 10, 2012</strong> -- Zimmerman's attorneys, Hal Uhrig (right) and Craig Sonner, announce that they will no longer be representing him.

  • Key Dates In The Trayvon Martin Case

    <strong>April 11, 2012</strong> - State Attorney Angela Corey announces the charging of George Zimmerman with second-degree murder. Zimmerman turns himself in to police and is booked into the Seminole County Jail. Mark O'Mara announces his role as Zimmerman's new attorney.

  • Key Dates In The Trayvon Martin Case

    <strong>April 23, 2012</strong> -- George Zimmerman's new lawyer, Mark O'Mara, enters a not-guilty plea on his client's behalf. Zimmerman is released from jail on a $150,000 bond. Per the conditions of his release, Zimmerman is required to wear a GPS monitoring device.

  • Key Dates In The Trayvon Martin Case

    <strong>April 24, 2012</strong> -- George Zimmerman shuts down his website. According to his attorney, the site raised $200,000.

  • Key Dates In The Trayvon Martin Case

    <strong>April 27, 2012</strong> -- Mark O'Mara launches the website GZLegalCase.com as the official site for Zimmerman's legal case.

  • Key Dates In The Trayvon Martin Case

    <strong>May 8, 2012</strong> -- At Zimmerman's arraignment, Seminole County Circuit Judge Kenneth Lester Jr. accepts his not-guilty plea.

  • Key Dates In The Trayvon Martin Case

    <strong>May 17, 2012</strong> -- Prosecutors release police reports, witness statements, surveillance videos and other evidence in the case.

  • Key Dates In The Trayvon Martin Case

    <strong>June 1, 2012</strong> -- Judge Lester revokes Zimmerman's bond, stating that his ruling is based on concerns that Zimmerman and his wife did not fully disclose their finances at the bond hearing.

  • Key Dates In The Trayvon Martin Case

    <strong>June 3, 2012</strong> -- Zimmerman is returned to jail.

  • Key Dates In The Trayvon Martin Case

    <strong>June 12, 2012</strong> -- George Zimmerman's wife, Shellie, is arrested on one count of perjury.

  • Key Dates In The Trayvon Martin Case

    <strong>June 20, 2012</strong> -- The Sanford city manager fires Bill Lee from the police force.

  • Key Dates In The Trayvon Martin Case

    <strong>June 21, 2012</strong> -- George Zimmerman's legal team releases discovery evidence on their client's website.

  • Key Dates In The Trayvon Martin Case

    <strong>June 29, 2012</strong> -- Zimmerman's second bond hearing is held. The judge does not immediately issue a ruling.

  • Key Dates In The Trayvon Martin Case

    <strong>July 5, 2012</strong> -- Judge Lester grants Zimmerman a higher bond of $1 million.

  • Key Dates In The Trayvon Martin Case

    <strong>July 6, 2012</strong> -- Zimmerman is again released from jail.

  • Key Dates In The Trayvon Martin Case

    <strong>July 19, 2012</strong> -- George Zimmerman relaunches his personal website.

  • Key Dates In The Trayvon Martin Case

    <strong>July 27, 2012</strong> -- George Zimmerman's wife pleads not guilty to perjury.

  • Key Dates In The Trayvon Martin Case

    <strong>Aug. 29, 2012</strong> -- An appeals court grants a request by George Zimmerman's defense team to dismiss Judge Lester from the case.

  • Key Dates In The Trayvon Martin Case

    <strong>Aug. 30, 2012</strong> -- Circuit Judge Debra S. Nelson is assigned the case.

  • Key Dates In The Trayvon Martin Case

    <strong>Oct. 19, 2012</strong> -- Judge Nelson grants a defense motion requesting access to Trayvon Martin's school records and social media posts. The state is also granted access to Zimmerman's medical records.

  • Key Dates In The Trayvon Martin Case

    <strong>Nov. 14, 2012</strong> -- Gov. Scott's "Stand Your Ground" task force concludes its final meeting and recommends no sweeping changes to the law.

  • Key Dates In The Trayvon Martin Case

    <strong>Nov. 20, 2012</strong> -- Former Casey Anthony attorney Jose Baez announces that he is representing Sanford police Detective Chris Serino, the lead investigator in the shooting.

  • Key Dates In The Trayvon Martin Case

    <strong>Dec. 3, 2012</strong> -- A new photo is released showing George Zimmerman with a bloody, broken nose on the night of the shooting.

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 5, 2013</strong> -- On this day, Trayvon Martin would have turned 18.

  • Key Dates In The Trayvon Martin Case

    <strong>Feb. 26, 2013</strong> -- Martin's parents hold a rally in his memory to mark the one-year anniversary of his death.

  • Key Dates In The Trayvon Martin Case

    <strong>March 26, 2013</strong> -- Zimmerman's defense team releases its witness list of 134 people, including Sanford police officers and 56 unnamed witnesses.