The madness, once again, descended without warning.

This time, instead of Virginia Tech or Columbine, 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 theater early Friday.

Authorities in Aurora, Colo., are just beginning to piece together how and why the suspected gunman, James Holmes, 24, allegedly killed at least 12 people and wounded 58 others during the premiere of a Batman sequel, "The Dark Knight Rises."

But emerging details suggested Holmes, a University of Colorado doctoral student in the process of withdrawing from his neuroscience program, was coming from an ominous place.

His apartment, about five miles from the Century 16 Movie Theater complex, was rigged with wires and incendiary materials, authorities said. Bomb technicians were trying to determine whether they were a hoax or posed a real danger.

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 theater, then left and returned through an emergency exit.

"We are not looking for any other suspects," Aurora Police Chief Dan Oates said. "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 Theater 9 at the multiplex.

Chris Ramos, 20, who was seated in the fifth row, said that about 20 minutes into the movie, someone at the front of the auditorium tossed what looked like stuffed toy baseball bats into the crowd. He said he thought the canisters were some sort of promotional gimmick for the film.

"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 was silent 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 the shooting continued for a minute and a half.

Salina Jordan, 19, was in an adjoining theater 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."

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 theater 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 Theater 9. Jordan said she watched a police officer carry out the body of a 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 theater.

Oates said Holmes was wearing a "ballistic helmet," a bulletproof vest, leggings, a throat protector, a groin protector, a gas mask and protective gloves.

The shooting, especially in Colorado, inevitably stirred memories of nearby Columbine High School and the mass murders there in April 1999, when two heavily armed students stalked through the hallways, killing 12 students and a teacher as they went, before shooting themselves.

Authorities began Friday evening to remove the bodies of 10 victims that remained inside the theater. Oates said police were working as quickly as possible to identify them and notify their relatives. Two other victims died in hospitals.

The Pentagon said three military personnel were injured in the shooting and that another service member at the theater remained unaccounted for. The Pentagon did not identify them or their units or provide details about their conditions.

Other victims remained hospitalized, with at least two in critical condition, and police warned that the death toll could increase. 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.

President Obama and his Republican presidential challenger, Mitt Romney, expressed condolences, canceled campaign events and suspended advertising in Colorado.

Noting that his two daughters like to go to the movies, just like millions of other Americans, Obama said that he and first lady Michelle Obama would "hug our girls tighter tonight."

The shooting is a "reminder that life is fragile," he said. "Our time here is limited, and it is precious."

Romney, in Bow, N.H., said he spoke "not as a man running for office but as a father and grandfather, a husband, an American. This is a time for each of us to look into our hearts and remember how much we love one another and how much we love and how much we care for our great country."

In Aurora, police converged on Holmes' apartment about 2 a.m. Friday, after he indicated it contained explosives.

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

A neighbor of Holmes' said she and other residents were awakened by police and told to get dressed and leave immediately. They did not explain why or mention the mass shootings, the neighbor said.