The Colorado shooter that killed 12 people in a theater was identified as James Eagan Holmes, according to various reports.

Witnesses described him kicking open the back door of the Century Aurora 16 theater and opening fire with four weapons, one AK-type assault rifle, a shotgun and two handguns, according to CNN. Wearing a bulletproof vest and goggles, he stood at the front of the theater, tossed tear gas into the crowd and starting randomly shooting.

The gunman left 12 people dead and around 50 people injured after opening fire in the Aurora theater, which was showing a midnight screening of "The Dark Knight Rises." Officials said Holmes acted alone when the shooting happened around 12:30 a.m.

The FBI revealed that Holmes is a 24-year-old white American male. He is 6 feet, 3 inches tall and was born Dec. 13, 1987, reported the Huffington Post.

One federal official told CBS News that at this time Holmes appears to have been "under the radar." CBS News reports Holmes doesn't appear to have a criminal record in Colorado or in California's San Diego County, where he lived previously. Pentagon officials said there is no record of Holmes having served in any branch of the military.

The Associated Press reported that Holmes was a student at the University of Colorado's medical school but withdrew last month, according to the school.

Holmes reportedly had Tennessee license plates on the vehicle he was driving at the time of the shooting, but a connection to that state remains unclear. He was arrested next to the vehicle, without incident, after the shooting spree ended, according to the Huffington Post.

Police, ambulances and emergency crews swarmed on the scene after frantic calls started flooding the 911 switchboard, officials said.

There was no immediate word of any motive. Federal law enforcement officials were briefed on the attack, but at this point there is no indication it is terrorism-related, CBS News reported.

Police evacuated the area surrounding Holmes' Denver-area apartment, saying the residence was apparently booby-trapped. A law enforcement source told CBS that police saw what looked like "buckets of extra ammunition" and some kind of chemical inside his home.

Aurora Police chief Dan Oates said it could take hours or days to safely enter the apartment.

According to Fox Business, Police started a search of Holmes' apartment around 2 a.m. Police begin searching a third-floor apartment in north Aurora using a police robot and camera attached to a long pole.

Wes Bradshaw and his mother Lavonne watched the search of the third-floor apartment from their apartment, they said. The two watched a police robot enter the building right before they heard a small explosion.

At least 20 were being treated at the University of Colorado Hospital, reported CNN. All of the wounded suffered from gunshot wounds, which ranged from minor to critical. The New York Daily News reported that the youngest victim was believed to be a three-month old infant, who was in fair condition, and a 6-year-old child was rushed to the hospital was a bullet wound.

A San Diego woman who identified herself as James Holmes' mother said she had no information about the shooting.

"You have the right person," the woman told ABC News. "I need to call the police. I need to fly out to Colorado."

CBS News reported that Benjamin Fernandez, who was also in an adjacent theater, said he saw a young girl, around 12 or 13 years old, laying on the concrete when he exited the building.

"From what I saw, she had two bullet wounds in her leg and just blood all over her stomach. Her friends were all just yelling, crying. ... We were just all pretty much in shock," he said.

Salina Jordan was in the next theater watching "The Dark Knight Rises" when she heard pops, the Washington Post reported.

"The part of the movie we got up to, they were shooting, and it was so in synch we thought it was part of the movie,'' said Jordan, 19, of Aurora. "We thought it was a special effect because they were trying to do it up big for opening night.''

Then bullets began piercing the wall of the second theater, Jordan said. A teenager to her right was shot in the jaw. The fire alarm went off and the public address system announced, "Emergency in the theater.''

Jordan said she saw four injured people being taken out of the theater, apparently alive, and watched as a police officer carried the inert body of a girl who appeared to be about nine years old.

"She was lifeless,'' Jordan said. "She had been shot through her stomach and the blood was just coming out. Her body was so limp. And her face, there was no life in her face.''

According to Newsday, President Barack Obama said he was saddened by the "horrific and tragic shooting," pledging that his administration was "committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded."