DENVER — Unhindered by federal background checks or government oversight, the 24-year-old 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 E. Holmes, 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 and 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.

Mr. 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 purchases. And neither Colorado nor federal law required him to submit to a background check or register his growing purchases, gun policy experts 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 that dealers track their ammunition sales for law enforcement.