* Former student charged with killing 12 during Batman film
* Prosecutors outline evidence against accused gunman
* Police describe nightmarish crime scene (Adds testimony about date of movie ticket purchase in seventh paragraph)
By Keith Coffman
CENTENNIAL, Colo. Jan 7 (Reuters) - The former graduate student accused of shooting a dozen people to death at a Colorado movie theater last summer sat impassively in court on Monday as police recounted the nightmarish mix of blood, tear gas, screams and flashing strobe lights they found in the darkened cinema.
Outside the crowded theater, police confronted the accused gunman, James Holmes, wearing a helmet, gas mask and head-to-toe body armor, the officers testified. He surrendered without a struggle, they said, and apart from smirking when asked whether he had acted alone, Holmes seemed "detached" from the mayhem around him.
Testimony from the first officers to reach the chaotic crime scene opened a hearing in which prosecutors set out to convince a judge that they have enough evidence to put Holmes on trial for one of the deadliest mass shootings in U.S. history.
The onetime University of Colorado doctoral student of neuroscience is charged with multiple counts of first-degree murder and attempted murder stemming from a shooting rampage that left 12 dead and 58 wounded at a midnight screening of the Batman film "The Dark Knight Rises."
The so-called preliminary hearing is expected to last a week and offer the public its first detailed look into the investigation of the July 20 carnage in Aurora, a Denver suburb.
It also will give Holmes' lawyers an opportunity to call witnesses to testify about his mental state. They are expected to mount an insanity defense if the case goes to trial.
In one piece of evidence suggesting considerable advance planning for the attack, police detective Matthew Ingui displayed security-camera footage of Holmes in the theater lobby before the shooting as he scanned a movie ticket the investigator said Holmes had purchased 12 days earlier.
Shackled and wearing a crimson jumpsuit, a bearded Holmes sat expressionless at the defense table. Dark brown hair has replaced the dyed bright red hair the California native had when he was arrested.
'TRAIL OF BLOOD'
Aurora Police Officer Jason Oviatt opened the testimony by recounting that he saw "a trail of blood that led to the theater" as he approached Holmes in the rear parking lot, initially mistaking the suspect for a fellow policeman.
Holmes "immediately put his hands up" when ordered to freeze, and he was handcuffed and taken into custody without resistance, Oviatt said.
Officer Justin Grizzle testified that Holmes kept silent after his arrest, even when asked if he had accomplices. "He didn't respond verbally. He looked at me and smiled," Grizzle recalled. "It was a smirk."
Authorities have concluded that Holmes acted alone.
As terrified moviegoers fled the theater, Oviatt said that Holmes appeared to be "very detached from it all."
Officers who rushed into the theater from the parking lot said they found dozens of people sprawled around the auditorium, some trying to crawl to exits over a floor slick with blood. The air was heavy with tear gas the assailant had released inside the theater.
Police described the macabre scene they encountered as the film projection continued while people moaned and cried for help, cell phones rang, and strobe lights from the auditorium's fire alarm system flashed.
Officer Grizzle choked back tears as he testified about how he tried to attend to the wounded before paramedics arrived, saying, "I didn't want anyone else to die."
In desperation, he and other police started transporting some of the wounded to hospitals in their patrol cars. Grizzle said he "could hear blood sloshing in the back of my car" after making four such trips.
The Aurora shooting ranked as the bloodiest instance of U.S. gun violence in 2012 until last month's massacre of 20 children and six adults at an elementary school in Newtown, Connecticut.
Few details about the crime and its immediate aftermath had been divulged by authorities before Monday.
Most of the evidence against Holmes has been sealed, and Arapahoe County District Judge William Sylvester has issued a gag order preventing all parties, including law enforcement, from discussing the case outside court.
Once the preliminary hearing ends, Sylvester will decide if there is sufficient cause for the case to proceed. If a trial is ordered, prosecutors would have 60 days from the time Holmes enters a plea to decide whether to seek the death penalty in the event he is convicted.
Legal analysts assume that Holmes will ultimately plead not guilty by reason of insanity, based on statements from his defense attorneys and court filings.
Public defender Daniel King has said that his client suffers from an unspecified mental illness. King has subpoenaed two witnesses for this week's hearing to testify about Holmes' state of mind before the massacre, according to court documents.
Authorities have said that Holmes bought a ticket to the Batman film, and that he left the theater minutes into the movie and propped open a rear exit door to allow himself a way back inside.
He then donned protective gear, armed himself with a shotgun, semiautomatic rifle and handgun, then returned to the auditorium moments later to spray moviegoers with gunfire, authorities said.
Officer Aaron Blue testified that just after Holmes surrendered, he blurted out that he had booby-trapped the house in which he lived with what Holmes called "improvised explosive devices." Bomb squad technicians disarmed homemade bombs without detonating any of them. (Reporting by Keith Coffman; Additional reporting by Laura Beth Coffman; Writing by Steve Gorman; Editing by John Wallace, Grant McCool, Kenneth Barry and Lisa Shumaker)