CENTENNIAL, Colo. — An FBI agent Tuesday described a labyrinth of wires and potential explosives that had been rigged to be set off by a tripwire at the Aurora apartment of James Holmes, who is accused of opening fire inside a crowded movie theater last summer, killing 12 people and injuring dozens of others.

The agent, Garrett Gumbinner, said 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 Holmes had gone 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,” Gumbinner said.

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

Defense attorneys say Holmes is mentally ill and have used their questions to try to make that point. They haven’t elaborated but plan to call at least two witnesses later in the week who could discuss Holmes’ mental health.

Among the materials that officers eventually found in Holmes’ apartment was a thermos that contained glycerin, which had been tilted at a 45-degree angle over a fry- ing 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 Holmes’ apartment building, Gumbinner testified.

On top of the trigger was a boombox, and atop that was a remote-controlled car, the agent said.

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

Holmes’ plan, 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 showed 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 getting his ticket by scanning his smart phone, 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, hide or run away in time.

As a police detective interviewed Holmes outside the theater, he started pretending that the paper bags on his hands — meant to preserve gunshot residue — were puppets.

Holmes tried to jam a staple into an electrical outlet. He played with a cup on the table. At least two officers noted that his eyes were dilated.

That description of Holmes after the July 20 attack, given Tuesday by police detective Craig Appel, seemed to undercut prosecutors’ attempts to show Holmes as methodical, spending two months to assemble his arsenal.

His first recorded purchase was two tear-gas grenades, ordered online May 10.

Holmes also bought two Glock handguns, a shotgun and an AR-15 rifle, along with 6,295 rounds of ammunition, targets, body armor and chemicals, prosecutors said.

He dyed his hair bright orange, then bought a scope and plastic bullets on July 1, both of which were documented in security video.

Finally, he purchased the glycerin and potassium permanganate — chemicals that could combine to create fire and sparks — from a Denver science store. At some point, he also improvised napalm, as well as thermite, a substance that burns so hot that water can’t extinguish the blaze.

Parts of Holmes’ carpet were soaked with gasoline and oil, and ammonium chloride, a white powder, was poured onto the floor in strips, Gumbinner said.

“It would have ignited, and the whole apartment would have exploded or caught fire,” Gumbinner said.

One initiating system was a model rocket-launch box that operated by means of infrared light, but Holmes told investigators that it wasn’t armed, Gumbinner said.

Police said Holmes volunteered information about the booby traps. Authorities went to the apartment and carefully dismantled them.

Prosecutors also used Holmes’ dating website profiles to try to prove that he knew the consequences of his actions. On two social networking websites — Match.com and FriendFinder.com — Holmes asked: “Will you visit me in prison?”

The Match profile was created in April; the FriendFinder account was opened on July 5. Holmes last accessed the sites two days before the July 20 shooting, detective Tom Welton testified.

Defense attorney Tamara Brady asked witness Steven Beggs, an agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, whether there was anything to prevent “a severely mentally ill person” from purchasing things such as chemicals, ammunition and handcuffs.

He replied no.

Defense attorney Daniel King asked Appel if Holmes was tested for drugs or other substances.

“I saw no indication that he was under the influence of anything,” Appel said.

If Holmes is found sane, goes to trial and is convicted, his attorneys can try to stave off a death penalty by arguing that he is mentally ill. Prosecutors have yet to decide whether to seek the death penalty.

If he’s found innocent by reason of insanity, he would likely be sent to the state mental hospital, not prison. Such a defendant is deemed innocent because he didn’t know right from wrong and is therefore “absolved” of the crime, said former Jefferson County District Attorney Scott Storey, who recently lost an insanity case.

Information for this article was contributed by Dan Frosch and Jack Healy of The New York Times; and by Dan Elliott,Thomas Peipert, Nicholas Riccardi and Colleen Slevin of The Associated Press.

Front Section, Pages 1 on 01/09/2013