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 theater 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."

But 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 five miles from the Century 16 theater, 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 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 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 Theater 9.

Chris Ramos, 20, a Starbucks barista 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 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 off the floor and out of the aisle. He estimated that the shooting continued for a minute and a half.

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. They directed frightened patrons to remain in place - or to run for the exits - as gunfire started and stopped.

Police almost immediately arrested Holmes, who was next to his white Hyundai outside a rear entrance to the theater. 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.

Authorities began early Friday evening to remove the bodies of 10 victims that remained inside the theater. Oates said police are working as quickly as possible to identify them and notify their relatives.

Victims remain hospitalized, with at least two in critical condition. 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. Those hospitalized ranged in age from 16 to 31.

President Barack Obama and his Republican presidential challenger, Mitt Romney, expressed condolences and canceled campaign events.

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

Romney, at an appearance in Bow, N.H., said "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."

Holmes, who will have a preliminary court appearance on Monday, had no previous contact with the Aurora Police Department save for a 2011 traffic summons for speeding, Oates said.

Although much of his background is still unknown, some initial details touched grimly familiar notes.

He was "very quiet, strangely quiet in class" and he seemed "socially off," said a University of Colorado neuroscience faculty member, who spoke on the condition of anonymity because of privacy concerns and said he taught Holmes in a class at the school's medical campus.

When the faculty member heard Holmes' name in connection with the shootings, he said he thought that the suspect could well be his student. He added that Holmes did very poorly on comprehensive exams last semester and that school officials contemplated putting him on academic probation.