Holmes set a second detonation system outside the building, the agent said.
Holmes, Gumbinner said, rigged one of the triggering devices, called a "pyro trip box" with a remote control. He then took the remote control outside and placed it on top of a white trash bag near the apartment building.
Next to the remote, Holmes placed a remote control car. Inside the trash bag, he put a portable stereo set to play 40 minutes of silence, followed by loud music, the agent said.
The plan, Gumbinner explained, was for someone to hear the music and be drawn to the remote control car with what appeared to be the remote control lying next to it. When that person picked up the remote to activate the car, he or she would have unknowingly triggered the explosion in the apartment.
Holmes also left rows of white powder on the floor, which Gumbinner said was ammonium chloride. The powder, Gumbinner believes, was meant "to scare us" and would have created a large amount of smoke if it had ignited.
Prosecutors showed several photographs of the devices in court.
Holmes legally bought thousands of bullets, four guns and chemicals, months before prosecutors say he opened fire on a crowded movie theater, an ATF agent testified today.
Earlier in the day, prosecutors played two 911 calls in court, including the very first call from movie goer Kevin Quinonez as the shooting was still underway.
At least 30 rapid-fire gunshots could be heard in the background of the 27-second call, along with screaming.
"Gunshots!" Quinonez can be heard saying.
The dispatcher pleads with Quinonez to give the theater address, but the sound of gunshots and chaos drowns him out.
"Say it loud," the dispatcher pleads before the call goes dead.
In a second call, Kaylan Bailey calls to say her two cousins, Ashley Moser and Veronica Moser Sullivan, have been shot. One is breathing and the other is not, she says.
"Are there officers near you?" the dispatcher asks.
Amid the noise and confusion, the dispatcher pleads with the Bailey to start CPR on 6-year-old Veronica Moser Sullivan, who has stopped breathing. Veronica later died.
Victims and families listening to the calls in the courtroom were weeping openly and holding hands. One woman buried her face in her hands. Holmes showed no emotion.
Aurora Police Detective Randy Hansen testified that the first 911 call was made that morning at 12:38 a.m. It was the first of 41 calls that came in, all within 10 minutes.
The preliminary hearing is essentially a mini-trial as prosecutors present witness testimony and evidence to outline their case against the former neuroscience student.
The hearing at the Arapahoe County District Court could last all week. Judge William Sylvester will decide whether the case will go to trial. Holmes' attorneys have not yet said whether they plan on using a insanity defense, in which case Holmes could possibly be deemed unfit to stand trial. Another possibility is that the hearing could set the stage for a plea deal.
If the case does not go to trial, this week's hearing could be the only opportunity for public testimony and release of information in a case where gag orders and sealed documents have kept much of the evidence and information away from the public.