DENVER — Unhindered by federal background checks or government oversight, the man accused of killing a dozen people inside a Colorado movie theater was able to build what the police called a 6,000-round arsenal legally and easily over the Internet, exploiting what critics call a virtual absence of any laws regulating ammunition sales.
With a few keystrokes, the suspect, James Eagan Holmes, 24, ordered 3,000 rounds of handgun ammunition, 3,000 rounds for an assault rifle, and 350 shells for a 12-gauge shotgun — an amount of firepower that costs roughly $3,000 at the online sites — in the four months before the shooting, according to the police. It was pretty much as easy as ordering a book from Amazon.
He also bought bulletproof vests, other tactical gear, and a high-capacity drum magazine large enough to hold 100 rounds and capable of firing 50 or 60 rounds per minute — a purchase that would have been restricted under proposed legislation that has been stalled in Washington for more than a year.
Holmes, a graduate student in neuroscience with a clean criminal record, was able to buy the ammunition without arousing the slightest notice from law enforcement, because the sellers are not required in most cases to report sales to law enforcement officials, even unusually large purchasess
And neither Colorado nor federal law required him to submit to a background check or register his growing purchases, gun policy specialists said.
A few states like Illinois, Massachusetts, and New Jersey, and cities like Los Angeles and Sacramento, have passed restrictions on ammunition sales, requiring permits for buyers or licenses for sellers, or insisting dealers track their ammunition sales for law enforcement.
But in Colorado, and across much of the United States, the markets for ammunition — online and in storefronts — are largely unregulated, gun-control advocates say.
Law enforcement officials have refused to say whether Holmes bought the ammunition from multiple sources or spaced out the purchases over several weeks to avoid drawing attention.
But as investigators combed through the contents of his apartment on Sunday — its explosive booby traps now defused — new details began to emerge of his activities in the weeks leading up to the rampage. They sketch a picture of man once captivated by the science of the human mind growing increasingly interested in weapons and how to use them.
On June 25, Holmes e-mailed an application to join the Lead Valley Range, prompting the owner, Glenn Rotkovich, to call back, more than once, to invite him to a mandatory orientation meeting. Nobody ever answered, but Rotkovich described the voice message as nearly incom-prehensible.
‘‘It was this very guttural, very heavy bass, deep voice that was rambling incoherently,’’ Rotkovich said. ‘‘It was bizarre on a good day, freakish on others.’’
Holmes never called back about joining.
In early July, Holmes ordered a Blackhawk Urban Assault Vest, a knife, and two magazine holders from a website called Tactical Gear, according to an order slip provided by the company’s chief executive, Chad Weinman. He chose expedited two-day delivery to his apartment in the eastern Denver suburb of Aurora, where the shootings took place early Friday, just a few miles from Holmes’s apartment.
‘‘I think it conveys a sense of urgency and shows premeditation,’’ Weinman said in an interview, adding that the company was ‘‘deeply saddened’’ its gear had apparently been used in a mass killing.
Three weeks after that purchase, stunned and bleeding witnesses outside the Century 16 multiplex in Aurora would describe how a man dressed in a black commando-style outfit and a gas mask strode into the theater where they were watching a midnight screening of ‘‘The Dark Knight Rises,’’ tossed some gas-spewing grenades into the packed auditorium, and opened fire.
Gun-control groups said the purchases of the ammunition demonstrated how easily anyone could build a veritable arsenal without attracting a whiff of attention from state or federal law-enforcement officials. Gun groups replied that stricter controls would not make the nation safer, but would only restrict constitutional rights.
Only a handful of states and cities have passed any laws requiring that gun dealers keep track of who is buying ammunition.
‘‘It’s a wide-open marketplace,’’ said Tom Mauser, a gun-control advocate in Colorado whose son was killed in the 1999 Columbine shootings. ‘‘The Internet has really changed things. You don’t have to show your face. It’s anything goes.’’
Some top law-enforcement officers were among those calling for more restrictions on ammunition sales.