Updated at 1:30 a.m. ET: AURORA, Colo. -- The booby-trapped apartment of mass shooting suspect James Eagan Holmes was "designed to kill," officials said Saturday as they described a complex operation to disarm sophisticated explosives without destroying evidence.
"This was certainly challenging," said FBI agent Jim Yacone , detailing the discovery of the explosive devices and the use of a robot to disarm a trip wire and a primary bomb.
"The threat has not been completely eliminated," Yacone said, while adding that enough progress had been made that "an extensive amount of evidence is in the process of being collected."
Aurora Police Chief Dan Oates told reporters that he believed the rigged apartment targeted the police. “We sure as hell are angry,” he said.
"This apartment was designed to kill whoever entered it," he said. "Make no mistake about it."
Holmes had received a "high volume of deliveries" over the last four months at home and at school, which would explain how he obtained thousands of rounds of ammunition, Oates said.
He said it could take another day to fully dismantle explosives in the apartment and collect evidence, adding that residents of four neighboring buildings could be allowed to return Saturday night. Residents of the building where Holmes lived might be allowed back on Sunday, he said.
Late Saturday afternoon, authorities removed all hazardous materials found in the apartment to a “remote undisclosed location” outside the city limits for disposal, police told NBC. Most of the items were chemicals that, if mixed together, would cause explosions, they said.
Escorted by police and fire crews, these materials were placed in a truck lined with sand and taken to an area east of Aurora, NBC affiliate KUSA reported.
Authorities counter-charged the material by exploding the chemicals and then placed the remains in a trench where diesel fuel was poured on top and ignited, according to KUSA.
The procedure burned up any explosive material that remained, KUSA reported, citing the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Some 30 aerial shells were among the items detected inside the apartment on Friday
Specialists earlier set off a small, controlled detonation inside the apartment after first disarming a trip wire and incendiary device.
The blast succeeded in disarming a second triggering device, Aurora police Sgt. Cassidee Carlson told reporters.
A siren and shouts of "Fire in the hole" preceded the blast. Live video showed the blast blowing out part of a window, but no smoke or fire resulted.
"The controlled detonation was successful," the Aurora Police Department said in a statement. "Still more work to be done in the apartment to include dealing with other devices. There is a possibility of more controlled detonations."
Three types of explosives
The 800-square-foot apartment appears to have three types of explosives, a law enforcement source told the Associated Press: jars with accelerants; chemicals that would explode when mixed together; and more than 30 "improvised grenades" that resemble commercially available aerial fireworks shells.
Before the detonation, Carlson announced a trip wire and explosive had been disarmed, saying "we have been successful in defeating the first threat."
She added that "another triggering mechanism" had been found inside, and that was later followed up with the controlled detonation.
"We're going to be very cautious," she added, because "we don't want to lose" any evidence.
Any undetonated explosives will be taken in trucks lined with sand to take the impact of any explosion, she said, adding that
Fire trucks surrounded the building on Saturday as a precaution. Some adults and children in nearby buildings not evacuated watched from balconies.
Earlier Saturday, streets around the building were cleared, four neighboring buildings were evacuated and some unattended vehicles were towed away.
On Friday, tenants in Holmes' apartment building cleared out, taking with them a few belongings.
The Red Cross said Saturday that 16 evacuees were in one of the agency's shelters.
The building is in a lower-income area about four miles from where the shootings took place during a midnight premiere of the Batman movie "The Dark Knight Rises".
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