Colorado police have entered the flat of James Holmes, the man suspected of shooting dead 12 people at a Batman film screening, and dismantled an incendiary device.
Police spokeswoman Cassidee Carlson said officers had “defeated the first threat” by disarming a trip-wire and an incendiary device.
Cassidee Carlson warned there were other devices, and bomb squad personnel later set off a small explosion at the building.
Some 58 people were also hurt in the attack at a Century 16 cinema in the Denver suburb of Aurora.
A man in a gas mask and body armor threw tear gas canisters at a midnight screening, then fired on the crowd in the cinema.
James Holmes, 24, was arrested outside the cinema, police said.
FBI agents and police discovered his flat was booby-trapped when they used a camera at the end of a 12-foot pole to look inside.
Technicians made a first attempt on Friday to disarm the traps, believed to include explosives, but withdrew when it became clear the property was too dangerous to enter.
Sgt. Cassidee Carlson said the device was set up to detonate when the first person entered the flat.
“We’ve defeated first threat. It was set up to kill, and that could have been police officers or anything,” she said.
“There are other devices inside, I’m not going into detail about what those devices are.”
Earlier, Aurora police chief Dan Oates said the apartment contained “things that look like mortar rounds”, adding: “I’ve personally never seen anything like what we’ve found in there.”
Residents were evacuated from the building and four other premises nearby.
Firefighters are monitoring the block of flats for gases in an effort to identify what chemicals might have been used in the booby-traps.
Hundreds of mourners joined a vigil near the cinema in Aurora on Friday night, leaving candles and flowers outside.
James Holmes was said to be armed with a rifle, a shotgun and two pistols when he launched his assault on the midnight screening of the new Batman movie The Dark Knight Rises.
He also bought 6,000 rounds of ammunition over the internet, Dan Oates said.
All weapons and ammunition were purchased legally within the past few months, he said.
Thirty people remained in hospital, Colorado Governor John Hickenlooper said, with 11 of those in a critical condition.
Authorities have established no terrorism link, nor any motive, and James Holmes had no criminal record other than a speeding fine.
James Holmes will appear at Arapahoe County District Court, in nearby Centennial, Colorado, on Monday at 09:30 local time.