AURORA, Colo. —

It was less than half an hour into a post-midnight screening of the latest Batman movie, “The Dark Knight Rises,” when a young man opened an emergency exit door and slipped into a packed multiplex theater. He was dressed in dark, head-to-foot body armor, including a helmet, gas mask, vest and throat guard, and he was armed.

“He didn't say anything,” said Tayler Trujillo, an 18-year-old moviegoer. “He like kicked the door open with his foot and held it open with his foot, and he threw something and it landed in the row in front of me.”

What ensued was several minutes of grisly horror as the intruder, armed with a combat-grade arsenal, set off two gas canisters and sprayed the theater with sustained gunfire. At least 12 people were killed and 58 others injured in a shooting that rekindled memories of the 1999 tragedy at nearby Columbine High School.

Witnesses described the gunman calmly shooting people throughout the theater at the Century 16 complex, seemingly at random, hitting men, women and children in the semidarkness as the movie continued to run behind him. It was a smoky, surrealistic, unimaginable scene, witnesses said, as moviegoers, some in costume for the Batman opening, realized that these bullets were real.

Seconds after the last shots were fired early Friday, police arrested a suspect, James Holmes, 24, whose only previous brush with the law appears to be a speeding ticket. Holmes, who was in the process of withdrawing from the University of Colorado, Denver's graduate program in neurosciences, left behind a booby-trapped apartment.

The suspect

Holmes came from a well-tended San Diego enclave of two-story homes with red-tiled roofs, where neighbors recall him as a clean-cut, studious young man of sparing words.

The son of a nurse, Arlene, and a software company manager, Robert, James was a brilliant science scholar in college.

The biggest mystery surrounding the 24-year-old doctoral student was why he would have pulled on a gas mask and shot dozens of people early Friday in a suburban Denver movie theater, as police allege.

In the age of widespread social media, no trace of Holmes could be found on Facebook, LinkedIn, MySpace, Twitter or anywhere on the Web. Either he never engaged or he scrubbed his trail.

A longtime neighbor in San Diego, where Holmes grew up, remembers only a “shy guy ... a loner” from a churchgoing family. In addition to playing soccer at Westview High School, he ran cross country.

The bookish demeanor concealed an unspooling life. Holmes struggled to find work after graduating with highest honors in spring 2010 with a neuroscience degree from the University of California, Riverside, said the neighbor, retired electrical engineer Tom Mai.

Holmes enrolled last year in a neuroscience Ph.D. program at the University of Colorado, Denver but was in the process of withdrawing, said school officials, who didn't provide a reason. The school later said in a statement that he left the program in June 2012.

The scene

The gunman had entered the theater through an emergency exit next to the screen, carrying his guns and canisters, witnesses said. “Everybody thought it was a joke,” said DeJonte Harris, 19, who was in the fourth or fifth row.

That ended when the assailant tossed the canisters, which police said contained either smoke or an irritant. Trujillo said the one near him “went off kind of like a firework, and gas filled the room, and right then all you heard was, 'Get down! Get down!' ”

“Then he fired a shot in the air,” recalled another moviegoer, Tre Freeman, 19. “And that's when all hell broke loose. He started shooting anybody and anyone; he just didn't care. We were just laying on the ground, praying that we weren't about to get shot.”

Harris ran past a woman curled up in a fetal position on the floor. He recalled looking back and seeing the gunman. “The last time I saw him, he was in front of the screen,” Harris said. “After that, I wasn't trying to see where he was at.”

In all, 70 people were killed or injured, according to Aurora Police Chief Daniel Oates. Ten died at the scene. In terms of the number of people hit by gunfire, it appears to be the largest mass shooting in U.S. history.

Oates said police seized four weapons and believe three were used in the assault.

Police received “hundreds” of 911 calls within moments after the shooting began at 12:39 a.m. The first officers arrived within 90 seconds of the first call, Oates said. Overwhelmed with bloodied victims, officers rushed the injured to hospitals in their patrol cars.

Police in Aurora evacuated five buildings in the vicinity of Holmes' apartment, which Oates said had been booby-trapped with incendiary and chemical devices looped together with wire. A bomb squad entered the apartment Friday but left after noticing a series of trip wires. Authorities worked into the evening trying to figure out how to disarm the trap.