He said Holmes also told him that he had left a boombox by a trash container outside his apartment rigged to start playing loud music 40 minutes after he turned it on.
Next to it, Holmes said, he left a remote-control toy car and a control device that would set off the explosives inside his apartment, Gumbinner testified.
Appel said someone took the boombox into an apartment where police recovered it and found Holmes' prints on it. They never located the toy car, he said.
Also, Steve Beggs, a supervisory agent for the Bureau of Alcohol, Tobacco, Firearms and Explosives, testified that Holmes had purchased 6,295 rounds of ammunition and four firearms beginning in May. Ten weeks before the attack, on May 10, he bought two canisters of tear gas over the Internet, Beggs said.
He was still buying materials into July, Beggs said, testifying that authorities have video of Holmes buying an accessory at a Colorado gun store on July 1. In the video, he said, Holmes' hair is bright orange.
Court room packed with family, spectators
In Monday's first day of testimony, police officers recounted arriving at the movie theater to find a detached, sweaty Holmes outside and a horrific scene inside the theater, where the floor had become slippery with blood and cell phones rang unanswered.
Holmes was a doctoral student in Aurora, in the neuroscience program at the Anschutz Medical Campus of the University of Colorado, Denver, until he withdrew a month before being arrested outside the bullet-riddled movie theater. He had been a patient of a University of Colorado psychiatrist, according to a court document filed by his lawyers.
Holmes did not speak during Monday's hearing. His bushy hair and long beard contrasted with the bright red hair and close-cropped facial hair he sported during previous appearances.
During portions of the hearing, family members of victims held one another, sobbing.
Security was tight. Spectators had to pass through a metal detector and then were searched again before entering the courtroom. At least nine armed officers stood guard inside, some of them scanning the audience packed with reporters and victims' family members.