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Colorado theatre shooting victim A.J. Boik’s mother (centre) is helped off the football field after family, friends and former classmates gathered for a memorial service at Gateway High School on Saturday in Aurora, Colorado. Image Credit: AFP

Colorado: Police in Aurora, Colorado on Sunday dismissed suspicions that another person helped 24-year-old James Holmes prepare the massacre at a movie theatre that left 12 people dead.

Authorities were still working to clear dangerous explosive materials from inside Holmes’ suburban Denver apartment a day after police said he opened fire and set off gas canisters in a suburban theatre minutes into the premiere of the Batman film The Dark Knight Rises. At least 70 people were wounded.

“There are multiple unconfirmed and inaccurate news reports about a second suspect,” police spokeswoman Cassidee Carson said in a statement.

“In regards to those reports: An associate of Holmes was interviewed this evening. There is no reason to believe that he is involved.”

She said further details of this investigation will not be released now.

Holmes planned the rampage with “calculation and deliberation”, police said on Saturday, receiving deliveries for months that authorities believe armed him for battle and were used to rig his apartment with dozens of bombs.

Meanwhile, a federal law enforcement official says the semi-automatic assault rifle used in the shooting jammed during the attack.

The official said late Saturday the rifle had a high-capacity ammunition magazine and that it jammed, forcing Holmes to switch to another weapon. The official spoke on condition of anonymity to discuss the investigation.

Aurora Police Chief Dan Oates has said a 100-round drum magazine was recovered at the scene of the shooting in suburban Denver. Oates said such a weapon would be able to fire 50 to 60 rounds a minute.

Oates said Holmes used a military-style semi-automatic rifle, a shotgun and a pistol to open fire on the unsuspecting theatre-goers. He had bought the weapons at local gun stores within the last two months. He recently purchased 6,000 rounds of ammunition over the internet, the chief said.

His apartment was rigged with jars of liquids, explosives and chemicals that were booby trapped to kill “whoever entered it”, Oates said, noting it would have likely been one of his officers.

“You think we’re angry? We sure as hell are angry,” Oates said.

Authorities wouldn’t discuss a motive for one of the deadliest mass shootings in recent US history, as makeshift memorials for the victims sprang up and relatives began to publicly mourn their loved ones. Holmes had recently withdrawn from a competitive graduate programme in neuroscience. Neighbours and former classmates in California have said he was a smart loner who said little.

But he had apparently prepared the attack at the Aurora theatre well in advance, receiving multiple deliveries by mail for four months to his home and school and buying thousands rounds of ammunition on the Internet, Oates said.

“He had a high volume of deliveries,” Oates said. “We think this explains how he got his hands on the magazine, ammunition,” he said, as well as the rigged explosives in his apartment.

“What we’re seeing here is evidence of some calculation and deliberation,” Oates added.

Federal authorities detonated one small explosive and disarmed others inside Holmes’ apartment after sending in a robot to take down a trip wire, FBI Special agent James Yacone said. Bomb technicians then neutralised what he called a “hyperbolic mixture” and an improvised explosive device containing an unknown substance. There also were multiple containers of accelerants, he said.

“It was an extremely dangerous environment,” Yacone said, saying anyone who walked in would have sustained “significant injuries” or been killed.

Holmes, 24, was in solitary confinement for his protection at a county detention facility on Saturday, held without bond on suspicion of multiple counts of first-degree murder. He was set for an initial hearing on Monday and had been appointed a public defender, authorities said.

Holmes also bought an urban assault vest, two magazine holders and a knife for just over $300 (Dh1,101) on July 2 from an online supplier of tactical gear for police and military personnel, according to the company.

Chad Weinman, CEO of TacticalGear.com, said his company processes thousands of orders each day, and there was nothing unusual in the one that Holmes placed. While his company often receives orders from military units and law enforcement organisations, it is not out of the ordinary for individual police officers or soldiers to place orders, he said.

“Everything Mr. Holmes purchased on July 2 is commercially available,” Weinman said, adding he was “appalled” that the material was sold to Holmes before the shooting.

It wasn’t known why the suspect chose a movie theatre to stage the assault, or whether he intended some twisted, symbolic link to the film’s violent scenes.

After buying a ticket to the movie, Holmes went into the theatre and propped open an exit door several minutes into the film, a federal law enforcement official said. The suspect then returned in protective gear and with high-powered weapons and opened fire, shooting scores of people and picking off victims who tried to flee, officials said.