The man suspected of shooting 12 people dead and wounding 70 during a screening of the new Batman movie in Colorado legally purchased four guns and 6,000 rounds of ammunition in the last 60 days, police have revealed.
More details about the bloody massacre at a cinema in Aurora, near Denver, – and the extent of the weaponry used - were outlined by officials at a news conference.
The gunman, who burst into a midnight screening of the firm and opened fire, was arrested shortly afterwards and has been identified as 24-year-old James Holmes, a loner who recently quit medical school.
Ten people were killed at the scene with another two dying later of their injuries.
Eleven people are still in a critical condition in hospital while only one of the dead has so far been identified .
Aurora police chief Dan Oates said Holmes had bought the four guns in local shops and the 6,000 rounds of ammunition over the internet.
All the weapons were purchased legally, he added.
Describing the rampage itself, he said: “As far as we know it was a pretty rapid pace of fire in that theatre.”
The gunman was wearing a gas mask and a bulletproof vest as he fired shots and hurled a gas canister at the movie-goers.
Holding back tears, an emotional Mr Oates also described the potential danger of booby traps the police face while trying to search the gunman’s flat.
His apartment is filled with trip wires and chemicals, leading to the evacuation of five buildings nearby.
The operation to search the flat would resume in the morning, Mr Oates said.
The police are also due to meet shortly with the victims' families.
The news conference came after Christopher Nolan, the director of the new Batman film that was being screened at the time of the massacre, called it a "senseless tragedy", that was "unbearably savage".
The Oscar-nominated filmmaker cancelled the French premiere of "The Dark Knight Rises" and a media junket in Paris hours after the shootings.
"Speaking on behalf of the cast and crew... I would like to express our profound sorrow at the senseless tragedy that has befallen the entire Aurora community," he said.
British-American Mr Nolan is director of the Batman trilogy that started with "The Dark Knight" in 2008 and of which "The Dark Knight Rises" is the final instalment.
US President Barack Obama said he was saddened by the "horrific and tragic shooting".
He cut short campaigning for the November presidential elections to return to the White House, where flags were lowered in mourning.
The Queen sent a message of sympathy to Mr Obama.
The British monarch said: "I was saddened to learn of the tragic loss of life earlier today in Denver, Colorado.
“Prince Philip joins me in extending our heartfelt sympathy to you and the people of America at this time."
But while the shooting side-tracked the US presidential race neither President Obama nor Republican challenger Mitt Romney addressed the volatile issue of gun control in America.
"There are going to be other days for politics," Obama said from one key electoral state, Florida. From another one, New Hampshire, Romney said much the same.
Amid their calls for unity and prayer, neither Mr Obama nor Mr Romney said anything of gun control, an issue that has been all but absent from the campaign debate this year.
Both men have shifted with the times, moving away from stances that favoured tougher gun control laws.
New York City Mayor Michael Bloomberg, a gun control advocate, said: "You know, soothing words are nice, but maybe it's time that the two people who want to be president of the United States stand up and tell us what they are going to do about it."
Twenty years ago, polls showed that a substantial majority of Americans - nearly 80% in 1990 - supported stricter limits on guns.
But now Americans appear evenly divided between those who want tougher restrictions and those who want to stick with current laws – which allow easy access to guns in many areas.