Aurora, Colorado (CNN) -- Kaitlyn Fonzi put her hand on the doorknob of the apartment upstairs from hers around midnight Thursday, irritated by the blaring electronic music inside.
The upstairs neighbor didn't answer when she called through the door. He'd cranked up the stereo, the deep bass of the music reverberating through the floor into the apartment Fonzi shared with her boyfriend.
The door felt unlocked, but Fonzi decided not to open it.
"I yelled out and told him I was going to call the cops and went back to my apartment," she told CNN.
It's a move for which Fonzi said Saturday she's "counting my lucky stars."
"It makes me feel extremely grateful, and it's definitely made me thank God, too, and thank just my instinct," Fonzi said.
Behind that door, investigators say, was a tripwire that would have touched off an array explosives and flammable liquids in the home of Colorado massacre suspect James E. Holmes. Anyone who crossed that threshold "would have sustained significant injuries or lost their life," Jim Yacone, the head of the FBI's Denver office, told reporters Saturday afternoon.
Holmes is being held as the suspect in the rampage at a nearby movie theater, accused of killing and wounding dozens at a screening of the new Batman film shortly after midnight Thursday. Investigators disarmed all the makeshift bombs he left inside his apartment on Saturday, using a "controlled detonation" to disable a second triggering device Saturday afternoon, said Sgt. Cassidee Carlson, an Aurora police spokeswoman.
"Although not certain, we are hopeful we have eliminated the remaining major threats," she said. "However, we will not know this until we enter the apartment."
Police evacuated Holmes' building and the four apartment blocks surrounding it after his arrest early Friday, when he told police he had rigged the flat with explosives. By 6 p.m. (8 p.m. ET), investigators had removed the remaining devices from the 850-square-foot apartment and hauled the explosives to an open field outside the city limits to be detonated, police said.
Residents of the surrounding buildings were allowed to return home Saturday evening, but it wasn't clear when Holmes' immediate neighbors could come back. It could be several days, Aurora Police Sgt. Tim Holt said. A shelter was set up at Aurora Central High School for those forced from their apartments.
It was not clear where all the explosives had been placed, though many have been seen in the living area with circuitry reaching into the kitchen, a law enforcement official with direct knowledge of the operation told CNN. One of the incendiary devices appeared to be improvised napalm and others appeared to be mixtures that, if combined with other materials, could cause an explosion, the official said.
"We talk about motive and defenses and deliberation, and make no mistake, this apartment was designed to kill whoever entered it," Aurora Police Chief Daniel Oates said. And who was most likely to enter that location? "It was going to be an officer."
The apartment also contained about 30 aerial shells used in fireworks displays, roughly the size of golf balls and filled with black powder, wired to a control box in the kitchen that was defused with water, another law enforcement official said. There were also jars of black powder, liquid accelerants and other flammable chemicals in the apartment, the source said.
The music had been on a timer and apparently started after the suspect left for the theater, according to another law enforcement source, who was not authorized to release details of the investigation to the media.
Fonzi said she called a non-emergency police number to complain about the noise after going to Holmes' door. A dispatcher told her they'd try to get an officer out to the apartment, but the noise kept up.
"I decided to call again a little bit before 1 o'clock, and they told me they were in an emergency state -- which I now know was the shooting -- and that they wouldn't be able to send anyone out there," she said.
The music stopped about 1 a.m. But within two hours, a SWAT team had kicked down the back door of the building and was banging on residents' doors, telling them to get out as soon as possible, she said.
Fonzi and her boyfriend have been staying with relatives since being evacuated. "We were considering moving prior to this, so maybe this is the push that we needed," she said.
She said she had last seen Holmes "a few months ago, just in passing."
"This really has told me that things like this can happen no matter how safe you feel, and just to make sure that everyone you love knows it," she said. "Be thankful for what you have, and trust your inner instincts."
Were you there or do you have thoughts about the shooting? Let us know on CNN iReport.
CNN's Jim Spellman, Susan Candiotti, Poppy Harlow, Chelsea J. Carter and Kyung Lah contributed to this report.