Police investigating the shooting spree at a screening of the new Batman film in Colorado are preparing to send in a robot to detonate what they called a sophisticated booby-trap in the apartment of the suspected gunman.
More than 24 hours after 12 people were killed and 58 injured at a cinema in Aurora, Denver, police were still trying to work out what the suspect, James Holmes, left behind.
The Aurora police chief, Dan Oates, said photographs had been taken of the inside of the apartment that appeared to show an elaborate trap. As a precaution nearby buildings were evacuated.
Oates said there were trip wires, jars of an unknown liquid and jars of ammunition piled up inside the flat. He described items that clearly resembled explosive devices.
Holmes, 24, a graduate student, was armed with an AR-15 assault rifle, a 12-gauge shotgun and a Glock .40-calibre handgun, Oates said. Police found an additional Glock .40 in his car, parked outside the cinema's rear emergency exit.
He had purchased the weapons legally at three local gun stores in the last 60 days and had bought 6,000 rounds of ammunition.
Officers who arrived on scene within 90 seconds of the first emergency calls took Holmes into custody in a car park behind the cinema, where he surrendered without a fight.
Police declined to say what, if anything, Holmes said to them after his arrest. Oates would not comment on possible motives.
Police said 30 people remained in hospital on Friday evening, 11 of them in critical condition. A makeshift memorial with 12 candles in a row and piles of flowers sat at a corner near the entrance to the car park. Up the hill from there, about 20 pastors led a vigil for about 350 people.
The cinema remained cordoned off with police vehicles lining the perimeter, and Oates said officers would be on hand for future showings of The Dark Knight Rises in the city.
In New York, police pledged to deploy officers at all 40 cinemas where the film was playing, partly as a precaution against copycats. Los Angeles police said they would increase patrols at screenings of the film.
The Paris movie premiere was cancelled on Friday. The film's director, Christopher Nolan, called the shooting "unbearably savage" and expressed "profound sorrow" for the victims and their families.
