"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.
Chaos broke out during the showing of "The Dark Knight Rises" at the Century Aurora 16 theater when the shooting began, police and witnesses said.
The gunman went to the rear door of the theater and propped it open, then tossed in a canister before starting to shoot, according to a federal law enforcement source involved in the investigation.
A federal law enforcement official told CNN the man used tear gas, but Oates said Friday afternoon that it was not clear what the substance was.
Oates said the man was wearing a ballistic helmet and protective gear for his legs, throat and groin, black gloves and a gas mask.
Jennifger Seeger, who survived the attack, said she had seen the man and thought his get-up was part of the entertainment for the film's debut.
She said the man first shot toward the ceiling, then began shooting at people. He reloaded during his attack, she said.
"He was just literally just massacring anybody that got up that was trying to run away," Seeger said.
The smoke smelled like a Fourth of July firework, said CNN iReporter Adam Witt. It took a few gunshots before he figured out what was going on.
"I hit the floor and hid behind the seats in front of me, pulling my wife down to hide with me," Witt said. "It was the longest minute of my life. The gunshots just kept coming. I knew it could be over any second. I knew my wife could be gone any second. It was absolutely surreal. I felt something hit my left arm, and my first thought was, 'At least it's just my arm.'"
"There were so many people running," he said. "I didn't look back. I just remember getting up from the floor and shouting, 'We have to run.'"
Witt said he held his wife's hand as they rushed out of the theater.
"There was a moment where I lost her hand, but I grabbed her shirt," he said. "We didn't let go of each other."
Quentin Caldwell, who was attending a screening in the adjacent theater, said he wasn't sure at first what was going on, despite hearing a "pop, pop, pop, pop" sound.
"We really didn't know something was happening until someone came from the left entrance and said we should not go outside because somebody with a gun was out there," he said.
Armed guards appeared at the theater exit and demanded audience members raise their arms to ensure they were not carrying weapons, then told them to run, Caldwell said.
"Outside was chaos. There was wounded everywhere," he said.
Officers rushed many of the wounded to hospitals in their patrol cars.
Police initially said 14 people had died -- 10 in the theater and four at area hospitals -- but revised the death toll to 12 later Friday morning, according to Aurora Police Lt. Jad Lanigan. The initial injury count of 38 was revised upward to 59 Friday afternoon.
Several people remained in critical condition at area hospitals.
Of the wounded, the University of Colorado Hospital said it had treated 22 people, including nine who were seriously injured. All of the wounded suffered from gunshot wounds, which ranged from minor to critical, spkeswoman Jacque Montgomery said.
The victims being treated there ranged in age from 3 months to 45, the hospital said.
Denver Health Center had six patients from the shooting, one in critical condition and five in fair condition, said Shelly Davis, house supervisor.
Swedish Hospital in Denver was treating three people, two of them in critical condition and one in fair condition. A fourth patient with minor injuries was treated and released, spokeswoman Nicole Williams said.
Children's Hospital Colorado said it had treated six adult victims, one of whom died. And Parker Adventist Hospital was treating two people for minor injuries, according to a spokeswoman.
It may take hours or even days to clear Holmes' apartment, Oates said.
Investigators looking into a window with a camera have seen a sophisticated booby-trap with trip wires connecting incendiary and chemical devices, he said.
Oates said he has never seen anything like it.
A woman who lives across the street from where the suspect's apartment said police evacuated her building around 4 a.m.
"They told us there was a bomb or bomb material located in the house across the street from us." Rebecca Bradshaw said.
Law enforcement officers who searched the suspect's apartment found "items of interest," a federal law enforcement source involved in the investigation told HLN. He did not elaborate.
Bureau of Alcohol, Tobacco and Firearms agents were involved in the apartment search, agency spokesman Tom Mangan said.
In addition to looking into the possibility of explosives, ATF agents also are conducting emergency traces on the weapons to see how they were obtained, Mangan said.
Authorities also searched the suspect's car in the parking lot of the movie theater.
In addition to canceling his Florida campaign appearances, Obama pulled some advertising in Colorado.
Presumptive GOP presidential nominee Mitt Romney also pulled advertising and issued a statement, saying he and his wife Anne were "deeply saddened by the news of the senseless violence."
Warner Bros., the studio behind the movie, said the company and filmmakers were "deeply saddened" to learn of the incident. The studio canceled the movie's Paris premiere, while New York police Commissioner Raymond Kelly said his officers would watch over screenings of "The Dark Knight" to prevent copycat shootings.
The National Theater Owners Association said in a statement that its members would be reviewing security procedures in the wake of the shooting.
The movie theater where the shooting took place has not been the site of any security incidents, said Timothy Warner, CEO of Cinemark Theaters, which owns the complex.
"You know, obviously you know, the person made a well-organized ... and had an assault weapon that would probably overpower any security that we would've had," he said. "I mean, this is obviously a very deranged gunman that had access to very high powerful weapons."
Aurora, a Denver suburb, is about 13 miles from Littleton, Colorado -- site of the April 1999 Columbine High School massacre.
In that incident, two teenage students, Dylan Klebold andEric Harris, armed themselves with guns and bombs and opened fire inside the high school. They killed 13 people and wounded 23 others before killing themselves.