US police have begun the intricate process of disarming booby traps in the apartment of the suspect behind the Colorado cinema shooting, hoping to find clues to the motive for the rampage without causing an explosion that could destroy key evidence.

Scores of law enforcement officials, including bomb squad technicians and dozens of federal agents, removed one trip wire and one explosive device inside James Holmes's apartment, and "other devices" are in there, Sergeant Cassidee Carlson said.

"We have been successful in defeating the first threat," she said, adding that the traps were meant to kill the first people entering the apartment in the Denver suburb of Aurora.

Holmes, 24, was arrested on Friday outside the suburban cinema with high-powered weapons and ammunition and charged with the rampage which killed 12 people and injured another 58 during the midnight showing of the new Batman movie, The Dark Knight Rises.

Makeshift memorials have sprung up to the victims, who included a US navy sailor, an aspiring sportscaster and a man celebrating his 27th birthday.

In his Saturday radio address, US President Barack Obama urged Americans to pray "for the victims of this terrible tragedy, for the people who knew them and loved them, for those who are still struggling to recover".

Seven of the wounded remain in a critical condition in hospital, some with injuries that could be permanent, a trauma surgeon said.

Police had delayed entering Holmes's apartment after learning it had been booby-trapped with trip wires and possible explosives, and evacuated several buildings around it.

Experts finally entered the apartment on Saturday and began to disarm the trip wires one by one to render them harmless, hoping not to detonate anything that could eliminate evidence against the suspect or information about a motive.

"We don't want to lose evidential value," Ms Carlson said. About 30 ammunition shells and up to 30 other devices in the apartment also need to be disarmed, she said.