The gunman who blasted his way through a packed movie house early on Friday, killing 12 people and injuring 58 others, had apparently been planning his attack for weeks.

"In the last 60 days he purchased four guns at local metro gun shops, and through the Internet he purchased over 6,000 rounds of ammunition," Aurora Police Chief Daniel Oates said Friday night.

The suspect, 24-year-old former medical student James Holmes, had also been stockpiling magazines to carry the gun ammunition. One clip recovered at the scene could hold 100 rounds for his military-style rifle.

"As far as we know, it was a pretty rapid pace of fire in that theater," said Chief Oates, noting that all the weapons purchases were legal.

Police are trying to learn more about the man's motives but have thus far been unable to access his booby-trapped apartment near the Colorado medical school where he recently withdrew from classes.

Investigators have described the apartment as being filled with chemicals, trip wires, jars of ammunition and possible mortar rounds.

"It is a very vexing problem how we'll get into that apartment safely," Chief Oates said. "I've personally never seen anything like what the pictures show us."

A law enforcement official familiar with the explosives left in the suspect's apartment described them as "pretty sophisticated" improvised explosive devices.

"These are not ordinary pipe bombs," said the investigator, who told Yahoo News on the condition his name would not be used. "No doubt it was meant to kill or maim any first responder that went into that apartment."

Officials said it could be Saturday or later before they get the suspect's home defused. If not, the investigator told Yahoo News, serious damage to the entire apartment building is a risk.

The thermal liquids left behind "could certainly heat up that room pretty quickly," he said. "We're not going to destroy a building."

Holmes is being held in the Arapahoe County Jail. Chief Oates said the man has an attorney, but refused to say if the alleged gunman is cooperating.

"I won't talk about his admissions," the police chief said. "The most important thing is that there is justice for these victims, and justice will occur in a courtroom."

Holmes is scheduled to make his first court appearance at 8:30 a.m. local time on Monday in Aurora.

Witnesses have given chilling descriptions of the scene at the movie theater.

"He was dressed in head-to-toe armor and looked like he meant business," said Jordan Crofter, who escaped the theater unharmed. "He just walked around like he was having fun. It was just target practice. His goal was to kill as many people as he could."

The number of casualties makes the incident one of the largest mass shootings in recent U.S. history.