Bomb experts triggered at least one small controlled explosion Saturday at the apartment of the alleged Batman gunman James Holmes, after de-activating a tripwire, witnesses said.

A small boom could be heard by reporters outside the apartment block, including AFP, where police have been trying to defuse multiple devices since the shooting which killed 12 people early Friday.

Others heard two or three. A short time later small parts of a blind could be seen floating in an open window on the third-floor apartment, through which police have been trying to determine exactly what is inside.

Five apartment blocks, including the one where Holmes lived, were evacuated shortly after he allegedly opened fire in a crowded theater showing an opening day midnight screening of "The Dark Knight Rises."

Twelve people were killed in the attack, and 58 injured in the hail of gunfire.

Police announced shortly before the controlled explosion that they had de-activated a tripwire.

"We have been successful in defeating the first threat, which includes defeating the tripwire and the first incendiary device," said Aurora Police Department spokeswoman Cassidee Carlson.

"This tripwire was set up to clearly detonate when somebody entered that apartment. It was set up to kill that person. That could have been a police officer executing a search warrant, or anything.

"So this is some serious stuff that our team is dealing with," she added.

With that achieved, the bomb squad experts are reassessing their next step, she said. A police statement added that, given the number of devices inside the apartment, they are not forecasting how long the operation will take.