"It's booby trapped with various incindiery and chemical devices and trip wires," Aurora police chief Dan Oates said, adding that it could take days to work through the apartment safely.
Five buildings around suspect James E. Holmes' Aurora apartment were evacuated, Oates said.
Police say Holmes, 24, dressed head-to-toe in protective tactical gear, set off two devices of some kind before spraying the theater with bullets from an AR-15 rifle, a 12-gauge shotgun and at least one of two .40-caliber handguns police recovered at the scene.
Oates said investigators are confident that Holmes acted alone.
The shooting unfolded inside a darkened theater packed with Batman fans, some in costume for the premiere of the movie"The Dark Knight Rises."Screaming, panicked moviegoers scrambled to escape from the black-clad gunman, who wore a gas mask and randomly shot as he walked up the theater's steps, witnesses said.
It was a scene "straight out of a horror film," said Chris Ramos, who was inside the theater.
"He was just literally shooting everyone, like hunting season," Ramos said.
Holmes surrendered without resistance within minutes of the first calls from panicked moviegoers reporting a shooting inside the Century 16 theater, Oates said. He is scheduled to appear in court on Monday, court officials said.
Police officers swarming to the theater encountered bloody, groaning victims streaming out of the theater. Others remained inside, many with gruesome injuries, according to recordings of emergency calls with dispatchers.
Victims flooded overwhelmed hospitals. One of the injured was just three months old, hospital workers said.
"I don't know how else to explain it. It's horrific," said Tracy Lauzon, director of EMS and trauma services at Aurora Medical Center.
Holmes is scheduled to appear in an Arapahoe County, Colorado, courtroom on Monday morning, Rob McCallum, spokesman for the Colorado Judicial Department, said Friday. The court file was sealed, according to a court order.
The FBI is assisting in the investigation, officials said.
FBI spokesman Jason Pack said it did not appear the incident was related to terrorism.
Colorado Gov. John Hickenlooper said the attack was the work of a "very deranged mind."
"Obviously no words can express the intensity of this tragedy," he said.
President Barack Obama canceled campaign events Friday, telling supporters at what had been scheduled as a rally in Fort Myers, Florida, that "there will be other days for politics."
"This will be a day for prayer and reflection," he said, calling for the country to unite as one and support the victims.
"Such violence, such evil is senseless. It is beyond reason," he said before ending the event to return to Washington.