01/08/2013 at 05:45 PM EST
Authorities for the first time Tuesday described the elaborate and deadly booby trap that James Holmes constructed in his apartment to blow up the building and delay police response to the massacre
he allegedly carried out at a packed Colorado movie theater.
Holmes, who had recently failed out of a graduate neuroscience program, used such items as a thermos, frying pan, fishing line, remote-control car and caustic chemicals to create a series of homemade bombs that never ignited, FBI bomb technician Garret Gumbinner testified in the second day of Holmes's preliminary hearing
Holmes, 25, was charged with 24 counts of murder
– two counts for each of the 12 people who died in
the July 20 theater attack in Aurora, Colo., which also injured at least 70.
He also had set a fishing line trip-wire from the door to a thermos of glycerine that he perched over a frying pan filled with potassium permanganate, Gumbinner testified.
On top of the refrigerator, investigators recovered a remote-controlled "pyrotechnic" box filled with 6-inch firework shells. Holmes left the remote outside, in a trash bag with a toy car and a portable stereo timed to start loudly playing music in 40 minutes, Gumbinner testified.
Holmes hoped that a neighbor would hear the music, open the bag, notice the car, handle the remote and detonate the explosives, Gumbinner said, recalling his interview with Holmes.
"He said he had rigged his apartment to explode or catch fire in order to send resources to the apartment, rather than the theater," Gumbinner said.
The agent also testified that there were roughly a dozen devices loaded with napalm, smokeless powder and live rounds. Carpets were soaked with oil and gasoline to enhance any blast.
But neighbors ignored the loud, Friday night music, avoiding the diversion Holmes planned. Detective Randy Hansen testified there were 41 calls made to 911 within the first few minutes of the shooting. Holmes stared straight ahead as the calls were played to the court, showing no emotion.
Upon his arrest outside the theater, Holmes told authorities about the explosives, and they used a robot controlled by a bomb technician to disable them.
Agent Steve Beggs testified that Holmes started stockpiling his arsenal on May 10, 2012, buying tear gas grenades online.
From then until July 14, Beggs testified, Holmes legally acquired nearly 6,300 rounds of ammunition, a .223 caliber semi-automatic rifle, a 12-gauge shotgun, two Glock .40-caliber pistols, bulletproof clothes, bomb-making material and handcuffs.