A total of 71 people were shot in the Colorado cinema during midnight screenings of the new Batman movie, police said.
Twelve people were killed, 10 of them at the cinema in Aurora, Denver, while another 59 adults and children were wounded.
Police arrested 24-year-old James Holmes, whose apartment four miles away was booby-trapped. Aurora Police Chief Dan Oates said bomb technicians are determining how to disarm flammable or explosive material in the third-floor apartment. He says police could be there some time.
Mr Oates said investigators are confident the gunman acted alone.
Mr Oates added Holmes wore body armour, used an assault rifle, a shotgun and a Glock handgun. His car was parked at the back of the cinema.
Mr Oates said there were four showings of the film at the time and all were sold out. Some movie-goers said they thought Friday's attack was part of The Dark Knight Rises, one of the most highly anticipated films of the summer.
Then they saw a silhouette of a person in the smoke at the front of the theatre, pointing a gun at the crowd.
"There were bullet (casings) just falling on my head. They were burning my forehead," said Jennifer Seeger, adding that the gunman, dressed like a SWAT team member, fired steadily except when he stopped to reload. "Every few seconds it was just boom, boom, boom," Seeger said. "He would reload and shoot and anyone who would try to leave would just get killed."
The Dark Knight Rises director Christopher Nolan called the Colorado cinema massacre a "senseless tragedy" and "devastating". He said films were a great American art form and watching one was an important and joyful pastime. He called the cinema his home, and said committing violence against innocent people there was savage and "devastating". He added that the cast and crew's thoughts were with the victims and their families.
It was the worst mass shooting in the US since an Army psychiatrist killed 13 soldiers and civilians at Fort Hood, Texas, in 2009, and brought memories of the massacre at nearby Columbine High School in 1999, where two students opened fire and killed 12 classmates and a teacher.