Police are trying to enter the Colorado shooting suspect's apartment but have been forced to move cautiously after spotting booby traps with trip wires.

Firefighters are monitoring the Aurora apartment building for gases in an effort to determine what chemicals 24-year-old James Holmes might have used to trap the place _ in case the materials go off, authorities said.

Police evacuated the building and surrounding residences after arresting Holmes as the suspect in a mass shooting Friday at a midnight screening of "The Dark Knight Rises." Twelve people were killed in the attack and about five dozen were injured, including 11 victims who remained in critical condition, authorities said.

FBI agents and police discovered it was booby-trapped when they used a camera at the end of a 12-foot pole to look inside.

"It's a pretty extensive booby trap. We're not sure what it's attached to. There are trip wires. There are three containers and we don't know what's inside," said Chris Henderson, deputy Aurora fire chief.

Police Chief Dan Oates said officers will try to enter on Saturday.

Photos of Holmes' apartment appeared to also show jars full of ammunition and liquid and other items unlike anything the chief had ever seen.

"I personally have never seen anything like what the pictures show us is in there," Oates said.

Friday evening police escorted residents individually and in pairs to their apartment units so they could quickly gather personal items.

Roberto Martinez, who lives in a building next to the suspect's building, hadn't been home since 4:30 a.m. Friday. He was escorted into his apartment and came out with a trash bag filled with items including toiletries, ice, a basketball and Air Jordan shoes.

He opted to stay in a hotel for the night instead of a shelter at a local high school, where some families with children were staying.

More than two dozen people were using a shelter at the Aurora Central High School Gymnasium.

Red Cross spokeswoman Melinda Epp said up to 30 residents were using the shelter to escape the heat, feed their children and wait out their exile. It was unclear how many people planned to spend the night there.

Kaitlyn Fonzi, 20, a graduate student at University Hospital, said she lives in the apartment below that of the suspect.

About midnight, Fonzi said she heard techno-like, deep-based reverberating music coming from that unit apartment. She went upstairs to the suspect's place and put her hand on the door handle. She felt it was unlocked, but she didn't know if he was there and decided not to confront him.

"I yelled out and told him I was going to call the cops and went back to my apartment," she said.

Fonzi called police, who told her they were busy with a shooting and did not have time to respond to a noise disturbance. She said she was surprised to learn later that the apartment was booby trapped and was shaken by the news.

"I'm concerned if I had opened the door, I would have set it off," she said.

Fonzi said she had seen the man one or two times before but never talked with him.

She said she believes the music was on a timer because it started about the time of the shootings.

Police have searched apartments and broken out windows at the building, but Fonzi said she doesn't know the condition of her apartment or car.

University of Colorado pharmacy student Ben Lung, 27, who lives two floors down from the suspect, said he and other residents were evacuated around 2 a.m. by armed SWAT officers armed with rifles.

"I heard a loud crash. It sounded like an air conditioner falling to the ground. About 10 minutes later, I heard police knock on my door. Police were armed with assault rifles and they brought us outside the apartment building and started questioning us," Lung said.

Lung said a few residents upstairs had called police around midnight and complained about loud music coming from the suspect's apartment.

Michelle Thuis, 26, who lives in an apartment near the entrance to the building, said police woke her up when they stormed in around 2:30 a.m.

"I heard them breaking down the front door. I called the police on them, then I looked out and saw it was the police," she said.

Thuis described the building as quiet and populated largely by students and doctors affiliated with a nearby University of Colorado Denver medical campus.

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Associated Press writer Mead Gruver contributed to this report.