On this day in 2012, a 24-year-old gunman goes on a rampage at a movie theater in the Denver suburb of Aurora, killing 12 people—the youngest a 6-year-old girl—and injuring at least 70 others. The shooting took place shortly after the start of a crowded midnight showing of “The Dark Knight Rises,” which opened across the United States that day. It was the deadliest mass shooting in Colorado since the 1999 massacre at Columbine High School, in which 12 students and a teacher were murdered before the two teen gunmen killed themselves.

The incident in Aurora began when the shooter, James Holmes, entered theater 9 at the Century 16 multiplex through a parking lot exit door and threw gas canisters into the theater. He was dressed in a gas mask and black combat gear, leading some audience members to initially think he was performing a stunt for the film, a Batman sequel highly anticipated by fans. Instead, Holmes opened fire at the audience, shooting people at random. Police quickly arrived on the scene, and Holmes was apprehended behind the movie theater; he put up no resistance. Not long after, law enforcement agents evacuated buildings near Holmes’ Aurora apartment after he told them he had booby-trapped his home with explosive devices. When Holmes made his first appearance in court, on July 23, his hair was dyed neon orange and he seemed dazed and devoid of emotion.

Investigators learned that in the months leading up to the tragedy Holmes had acquired weapons from Colorado gun shops and ordered thousands of rounds of ammunition online. A native of San Diego, he had enrolled in a Ph.D. neuroscience program at the University of Colorado Anschutz Medical Campus in Aurora in 2011 but dropped out in June 2012 after failing an important exam. Court documents later revealed that a month before the shooting, a University of Colorado psychiatrist who had treated Holmes reported to campus police that he could be a danger to the public and had threatened her.

Holmes, who has offered no motive for the shooting spree, eventually was charged with 166 counts of murder, attempted murder and weapons charges. In May 2013, he pleaded not guilty by reason of insanity. In November of that same year, a judge indefinitely postponed Holmes’ trial so lawyers on both sides can debate whether he should undergo further psychiatric evaluation.