CENTENNIAL — Just after midnight, the boombox would blare the music.

The music would draw the attention. The attention would cause someone to jostle the door. The door would tug the fishing line. The line would topple the thermos full of glycerin, and the glycerin would mix in the frying pan with the potassium permanganate to create a flame.

The flame would set the petroleum-soaked carpet on fire. The fire would light the fuses. The fuses would detonate the jars filled with either homemade thermite, smokeless powder or stove-top napalm mixed with bullets.

The nonexplosive ammonium chloride powder sprinkled across the floor by the front door was there just to create smoke and scare the police.

And that, FBI agent Garrett Gumbinner testified Tuesday in the second day of preliminary hearings in the Aurora theater massacre, was one of two active — but untriggered — explosive systems that detectives found inside suspect James Holmes’ apartment.

“The whole apartment would have either exploded or caught fire,” Gumbinner said.

After an opening day of dramatic testimony about the July 20 shooting that left 12 dead and 58 wounded, prosecutors and defense attorneys focused their attention Tuesday on the crux of the case: Is Holmes a calculated killer or mentally ill?

Through the testimony of several investigators, prosecutors spent the day seeking to show a depth of forethought and planning to the attack. And, in cross-examination, defense attorneys hinted even more openly that they intend to argue his actions were driven by mental illness.

The hearing — which serves the relatively banal purpose of determining whether there is enough evidence against Holmes for him to face trial on 166 counts of murder, attempted murder, explosives possession and committing a crime of violence — could end as early as Wednesday. Prosecutors said Tuesday afternoon that they are on their last witness.

Gumbinner was one of two detectives who interviewed Holmes the afternoon following the early-morning shooting, hoping to learn how to safely defuse the explosive tangle they say he created.

“He said he rigged his apartment to explode and catch fire,” Gumbinner said of Holmes. “He said he was hoping to send resources to his apartment rather than the theater. By resources, I mean law enforcement.”

In addition to Gumbinner, an Alcohol, Tobacco, Firearms and Explosives agent detailed the 16 separate purchases Holmes is accused of making to acquire the four guns; 6,295 rounds of rifle, shotgun and handgun ammunition; dozens of gun magazines; ballistic gear; weapon slings; two laser sights; two tear-gas canisters; hundreds of practice targets; explosive chemicals; fireworks supplies; handcuffs; and a military first-aid dressing.

ATF agent Steven Beggs said the purchases — all from online retailers or the Gander Mountain and Bass Pro Shops sporting goods stores in the metro area — began May 10 and ended July 14. There were five purchases in the first week of July alone.

An Aurora detective also testified about two online dating profiles Holmes placed on the websites Match.com and Adult Friend Finder. Detective Tom Welton said Holmes, in both profiles, asked the same question: “Will you visit me in prison?”

But, as prosecutors sought to establish premeditation in the attack, Holmes’ attorneys also revealed more of their strategy.

After prosecutors finished questioning Beggs, defense attorney Tamara Brady asked him whether it was illegal, for instance, to buy body armor or tear gas. Beggs said it wasn’t in Colorado.

“Is there any process in place in Colorado,” Brady persisted, “to screen out whether a severely mentally ill person is purchasing these items?”

Beggs said no.

In afternoon testimony, Aurora homicide Detective Craig Appel testified that police did not request a blood sample from Holmes after he was arrested. He said Holmes’ pupils were wide, but that “I saw no indication that he was under the influence of anything.”

But under defense questioning, he said Holmes’ behavior was at times odd while he was under observation.

Paper bags had been placed over his hands to preserve evidence and Holmes played with them as though they were puppets, Appel said. He also tried to place a staple into an electrical socket, he said.

Throughout Tuesday, there were reminders of the human tragedy that brought Holmes and the victims’ families together in the same courtroom.

Early in the day, prosecutors played a 911 call from a panicked 13-year-old girl.

Two of her cousins — police later determined they were Ashley Moser and Moser’s 6-year-old daughter, Veronica — had been shot. They needed help, the girl told the dispatcher between sobs. The noisy chaos of the theater filled the background.

“Kneel beside your cousin,” the dispatcher instructed the girl.

“I can’t hear you,” her voice pleaded back. “I’m so sorry.”

The girl began to cry.

“Ma’am, we need to start CPR,” the dispatcher began again.

“I can’t,” she said.

“I will walk you through the steps,” the dispatcher said.

The call ended with police arriving. They could not save Veronica.

Tuesday’s portion of the hearing concluded with Aurora Sgt. Matthew Fyles on the witness stand, going name by name through a list of wounded victims and detailing their injuries.

Two were paralyzed.

One lost his leg.

One was eviscerated but lived.

One suffered permanent brain damage.

One miscarried after the shooting.

Two others were pregnant and have since given birth.

When prosecutor Karen Pearson reached her question about Ethan Rohrs, who was 4 months old when the shooting occurred, she paused.

“Was Ethan Rohrs a baby?” she asked Fyles.

“Yes,” replied Fyles, his voice worn with emotion from reading the list. “He still is.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold