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The agent, Garrett Gumbinner, said that Mr. Holmes told the authorities after his arrest that he had hoped an explosion at his booby-trapped apartment would divert law enforcement officers from the movie theater, where Mr. Holmes had gone armed with a semiautomatic rifle and two other firearms.

"He said he had rigged his apartment to explode or catch fire in order to send resources to the apartment rather than the theater," Mr. Gumbinner said.

The agent's testimony came during the second day of a preliminary hearing to determine whether there is sufficient evidence to try Mr. Holmes, 25. He faces more than 160 counts of murder and attempted murder.

Among the material officers eventually found in Mr. Holmes's apartment was a thermos that contained glycerin, which had been tilted at a 45-degree angle over a frying pan containing potassium permanganate — which had the potential to cause intense heat, and possibly, a fire. The authorities also found three jars filled with improvised napalm, smokeless powder and live rounds.

The explosives had been attached to a remote-controlled trigger that was placed in a white trash bag inside a garbage can outside of Mr. Holmes's apartment building, Mr. Gumbinner testified.

On top of the trigger was a boom box, and atop that was a remote-control car, the agent said.

The boom box played 40 minutes of silence, then played music, he said.

Mr. Holmes's aim, said the agent, was that someone would hear the music and end up playing with the remote-control car, which could have triggered an explosion in his apartment.

During Monday's proceedings, video images from a security camera played in court here showed Mr. Holmes, a former neuroscience graduate student, walking into the busy movie theater and politely holding the door open behind him for an arriving couple. Next, he is seen retrieving his ticket by scanning his smartphone, and then lingering at a popcorn counter before he heads toward Theater 9, where the authorities say he opened fire, indiscriminately gunning down whoever was unlucky enough not to be able to duck or hide or run away in time.

Responding police officers said that when they first encountered Mr. Holmes outside the movie theater wearing a gas mask and helmet, they thought he might have been a fellow officer.

But soon enough, they detected something wrong, not least that Mr. Holmes was apparently trying to conceal a handgun.

"It was like there weren't normal emotional responses," said Officer Jason Oviatt of the Aurora Police Department, who handcuffed Mr. Holmes moments after the shooting. "He seemed very detached from it all."