The madness, once again, descended without warning. This time, instead of Virginia Tech, Columbine or Fort Hood, it was a suburban Denver multiplex where a heavily armed man clad in black came through an emergency entrance, set off canisters of an unknown gas and opened fire in a darkened theatre early Friday morning.

Authorities in Aurora, Colo., are just beginning to piece together how and why the suspected gunman, James Holmes, 24, allegedly killed 12 people and wounded 59 others during the midnight premiere of The Dark Knight Rises.

One of the dead was Jessica Ghawi, an aspiring sportscaster who narrowly missed being in the Eaton Centre food court during the fatal shooting there last month.

Emerging details suggested that Holmes, a University of Colorado graduate student who was in the process of withdrawing from his neuroscience program, was coming from an ominous place. His apartment, about six kilometres from the Century 16 theatre, was rigged with wires and incendiary materials, authorities said. Bomb technicians were trying determine whether they were a hoax or posed a real danger.

Aurora Police Chief Dan Oates said in a news conference that the investigations at the apartment would be put on hold overnight and resume Saturday.

The only near-certainty Friday was that the gunman had acted alone and not as part of a terrorist group or other conspiracy. Federal law enforcement sources said that Holmes bought a ticket, entered the theatre, then left and returned through an emergency exit.

“We are not looking for any other suspects,” Oates told reporters. “We are confident that he acted alone, but we will do a thorough investigation to make sure that is the case.”

Witnesses recounted scenes of chaos and bloodshed inside Theatre 9.

“The first sign that something was wrong was when the guy next to me got shot,” said Ramos, who attended the premiere with his sister and two friends. “I shielded my 17-year-old sister on the floor. I started crying, not because I was afraid, but because the tear gas started to burn my eyes.”

The gunman looked calm and uttered not a word as he walked up an aisle, firing as he went, witnesses said.

It was complete panic as survivors pushed to reach the exits, Ramos said, adding that he was kicked in the face several times by people trying to get up off the floor and out of the aisle. He estimated that the shooting continued for a minute and a half.

Salina Jordan, 19, was in an adjoining theatre watching the same movie when she heard a series of pops.

“It was so in sync we thought it was part of the movie,” she said. “We thought it was a special effect because they were trying to do it up big for opening night.”

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Then bullets began piercing the wall. A teenager to Jordan’s right was shot in the jaw. A fire alarm went off.

Officers arrived at the theatre complex within 90 seconds of receiving the first 911 call at 12:39 a.m., authorities said.

In the lobby, near the concession stands, SWAT teams trained their guns on Theatre 9. They directed frightened patrons to remain in place — or to run for the exits — as gunfire started and stopped. Jordan said she watched a police officer carry out the inert body of a young girl, who appeared to be about 9.

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

Police almost immediately arrested Holmes, who was next to his white Hyundai outside a rear entrance to the theatre. Oates said he was wearing a “ballistic helmet,” a bulletproof vest, leggings, a throat protector, a groin protector, a gas mask and protective gloves.

Federal law enforcement sources said that all four guns they think were used in the attack — two Glock pistols, a Remington 12-gauge shotgun and a Smith & Wesson AR-15 assault rifle — were purchased legally over the past two months from the local branches of two national chains: Gander Mountain and Bass Pro Shop.

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Authorities began early Friday evening to remove the bodies of 10 victims that remained inside the theatre. (Two of the 12 victims died later in hospital.) Oates says officers expect to get a confirmed list of the deceased and meet with their families Friday night.

The Pentagon said that three military personnel were injured in the shooting and that another service member at the theatre remained unaccounted for.

Eleven of the 70 people injured in the shooting rampage remained in critical condition on Friday evening, police said. Thirty were still hospitalized, Oates said.

James Denton, trauma director for the Medical Center of Aurora, said 12 patients were admitted with gunshot wounds and three were treated for chemical exposure. The victims ranged in age from 16 to 31.

In Aurora, police converged on Holmes’ apartment, in a modest neighbourhood about six kilometres from the theatre, about 2 a.m. Friday, after he indicated that it contained explosives.

Oates initially told reporters that the apartment, No. 10, appeared to be rigged with “pretty sophisticated” explosives. But at a news conference later, he said that bomb technicians were trying to determine whether the maze of wires and incendiary materials was a legitimate danger.

Kaitlyn Fonzi, 20, said she lives in the apartment below the suspect’s. About midnight on Friday, she said she heard techno-like, reverberating music coming from his unit. She went upstairs to the suspect’s place and put her hand on the door handle. She felt it was unlocked, but she didn’t know if he was there and decided not to confront him.

“I yelled out and told him I was going to call the cops and went back to my apartment,” she said.

Fonzi called police, who told her they were busy with a shooting and did not have time to respond to a noise disturbance. She said she was shaken to learn later that the apartment was booby trapped.

“I’m concerned if I had opened the door, I would have set it off,” she said.

She said she believes the music was on a timer because it started about the time of the shootings.