Police in Aurora, Colorado, detonated a device while disarming the apartment of James Holmes, the suspect in the movie-theater shooting that killed 12, causing a blast that sent smoke and debris flying.
Officers were entering the booby-trapped apartment to remove explosives that Mr Holmes (24) rigged before the shootings, said Sgt Cassidee Carlson of the Aurora Police Department.
She said police were proceeding cautiously as Mr Holmes was sophisticated and deliberate. A tripwire was found at the his home's entrance in the three-story building, she said.
"It clearly was intended to kill," she said. "This is serious stuff."
At about 11.40 am local time (6.40pm Irish time), a fire truck raised its bucket to the window and two helmeted bomb squad members placed a device inside.
The bucket retracted, a fire engine blasted its horn three times and the explosion went off. Debris flew from the window into a parking lot.
The Aurora Police Department posted on Twitter: "The controlled detonation was successful." Streets nearby were reopened after the detonation.
Police planned to remove about 30 devices inside the apartment and an additional 30 artillery shells, they said.
Meanwhile, a memorial of flowers and candles has been set up at the Aurora shopping mall where the shooting rampage at a showing of The Dark Knight Rises turned a movie screening into a chaotic scene of dead or bleeding victims.
A handwritten sign read: "7/20 gone not forgotten."
The gunman - armed with an assault rifle, a shotgun and a pistol, and wearing a full suit of tactical body armour, a helmet and a gas mask - set off two smoke bombs before opening fire in the dark theatre.
Police said 30 people remained hospitalised today, 11 of them in critical condition.
Officers who arrived on scene within 90 seconds of the first emergency calls quickly took Holmes (24) into custody in a parking lot behind the cinema, where he surrendered without a fight, Aurora Police Chief Dan Oates said.
Mr Holmes, a graduate student who authorities said had his hair dyed red and called himself “the Joker” in a reference to Batman's comic-book nemesis, was due to make an initial court appearance on Monday.
Authorities were unable to enter Mr Holmes' apartment, on the top floor of a three-story building, saying he had booby-trapped it with what appeared to be sophisticated explosives.
Police declined to say what, if anything, Mr Holmes said to them following his arrest.
During an emotional press conference, Mr Oates would not comment on possible motives for the massacre that stunned the community and the nation.
US president Barack Obama called the shootings a reminder that life is fragile and promised that the federal government stood ready to do all it could to seek justice for the "heinous crime."
"Even as we come to learn how this happened and who's responsible, we may never understand what leads anyone to terrorize their fellow human beings," Mr Obama said in his weekly radio and internet address, which was broadcast today.
Witnesses at the movie theatre told of a horrific scene, with dazed victims bleeding from bullet wounds, spitting up blood and crying for help.
Among those taken to hospitals as a precaution was a baby boy just a few months old.
"I slipped on some blood and landed on a lady. I shook her and said, 'We need to go; get up,' and there was no response, so I presumed she was dead," said Tanner Coon (17).
The suspect may have blended in with other moviegoers who wore costumes as heroes and villains, and some witnesses said they believed at first that his appearance was a theatrical enhancement to the film.
"It was just straight chaos," said Jennifer Seeger (25).
"Everybody was starting to scream and run at that point. He went straight from here to here with a gun in my face at that point. That rifle was in my face and I honestly didn't know what to think."
One of the victims was a sports blogger who wrote of surviving a shooting in Canada just over a month ago.
Jessica Ghawi, also known as Jessica Redfield, had blogged at length about surviving the June 2nd Eaton Centre mall shooting in Toronto that killed two people and injured several others.
Her brother Jordan Ghawi said his sister’s death was a “complete and utter shock”. He had been using his blog and Twitter account to update what he knew about his sister’s condition throughout the ordeal and also appeared on the Today Show.
Mr Ghawi said on his website that a man who was with his sister at the cinema described the chaos, saying he and Ms Ghawi (24) dropped to take cover when the gunman first started shooting.
The shooting evoked memories of the 1999 massacre at Columbine High School in Littleton, 27km from Aurora, where two students opened fire and killed 12 students and a teacher.
It also resonated in the US presidential race. Both Mr Obama and his Republican rival, Mitt Romney, toned down their campaigns, pulled their ads from Colorado and dedicated their scheduled events to the victims.
The gunman was armed with an AR-15 assault rifle, a 12-gauge shotgun and a Glock .40-caliber handgun, Mr Oates said.
Police found an additional Glock .40-caliber handgun in his car, parked just outside the theatre's rear emergency exit, he said.
Mr Holmes had purchased the weapons legally at three area gun stores in the last 60 days and had bought 6,000 rounds of ammunition, Mr Oates said.
A law enforcement official who asked to remain anonymous said the suspect had purchased a ticket, entered the theatre and propped open the emergency exit while he slipped out to "gear up" and return armed.
The portrait of Mr Holmes that emerged in the hours following the shooting remained fuzzy, with only a speeding ticket on his record and nothing to suggest he was capable of an outburst of gun violence.
Mr Holmes' family issued a statement of sympathy for the victims, saying: "Our heart goes out" to their loved ones, while they also asked for privacy from the media while they "process this information."
Agencies