AURORA, Colo. >> Colorado firefighters are monitoring an apartment building for gases in an effort to determine what chemicals one of its residents might have used to booby trap the place in case the materials go off, authorities said Friday.

The resident, James Holmes, 24, is the suspect in a mass shooting early Friday at a movie theater about four miles away.

“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 it’s inside,” said Chris Henderson, deputy Aurora fire chief.

If there is a detonation that causes a fire, firefighters will fight it from the outside of the building, he said.

The building and several around it have been evacuated.

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 like 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.