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Debris flies out a window after authorities detonated an explosive device inside the apartment of shooting suspect James Holmes on Saturday.

AURORA, Colo.—The alleged gunman behind the movie-house shooting that killed 12 people and wounded 58 others Friday had a "high volume of deliveries" by mail to his home and school over the past four months as he built an arsenal of weapons and explosives to conduct attacks, authorities said Saturday.

"We think this begins to explain how he got his hands on all of the magazines and ammunition" Aurora Police Chief Dan Oates said of the alleged shooter, James Holmes. "We also think it begins to explain some of the materials that he had in his apartment."

Dozens of explosive devices, including more than 30 homemade grenades, were among the dangers found lurking in Mr. Holmes's booby-trapped apartment, police said.

Mr. Oates also said that the elaborate tangle of explosive booby traps in Mr. Holmes's apartment appeared to be targeting police.

"Who was most likely to enter that location" after the shooting, Mr. Oates asked. "It was going to be a police officer."

The devices at the apartment included a tripwire at the front door. Most of the traps were disarmed with help of a mechanical robot, although authorities stressed that the apartment was still not entirely safe as of Saturday afternoon.

Local, state and federal investigators worked to unravel the web of traps and explosives in the apartment Saturday and were able to preserve samples of many of the substances Mr. Holmes had inside. They are being sent to a federal laboratory in Quantico, Va. for analysis, said Jim Yacone, the special agent in charge of the Federal Bureau of Investigation's Denver division.

"It was an extremely dangerous environment," Mr. Yacone said. He said if a neighbor or first responder had walked in the door, they would have been badly hurt or killed.

Authorities tried to alleviate concerns that the neuroscience student had obtained materials from a school laboratory, saying that a search of the lab's inventory had found nothing missing.

Associated Press

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"There was no evidence that he had access to any dangerous materials," Mr. Oates said.

For several hours Saturday, bomb experts painstakingly disarmed the traps in the apartment, including performing controlled detonations of two devices.

In addition to the shells found in the apartment, jars of a kind of accelerant were also found inside the dwelling, said Aurora Police Sgt. Cassidee Carlson, a department spokeswoman.

Mr. Holmes had purchased two firearms at a Bass Pro Shop in Denver, the company said Friday.

Mr. Oates said Friday that the suspect bought four guns over the past 60 days, and over the Internet bought 3,000 rounds for an assault rifle, 3,000 rounds for Glock handguns and about 300 shotgun rounds. The suspect also had a 100-round drum-style magazine for the assault rifle that would have allowed him to fire 50 to 60 rounds a minute.

The apartment is on the top floor of an aging three-story redbrick building, which has been evacuated since early Friday morning. Several neighboring buildings were also evacuated, although police said Saturday that residents of those buildings might soon be able to return.

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