NEW YORK (AP) -- Thousands of people with criminal records could illegally purchase guns on just one classified ad website, according to an investigation commissioned by New York Mayor Michael Bloomberg.

Bloomberg's group, Mayors Against Illegal Guns, conducted a four-month investigation of Armslist.com, which he dubbed "the Craigslist for national gun sales."

The pro-gun control group combed the public ads from people looking to purchase firearms. Using the contact information posted by the would-be buyers, Bloomberg's advocacy group secretly ran background checks of about 600 people and found that 1 in 30 of them had a felony record, which excludes them from buying firearms.

Bloomberg's group then extrapolated the data to conclude the site could have more than 25,000 illegal buyers in a year.

"This investigation has just scratched the surface of the Internet's vast illegal gun market — and if nothing is done, that market will continue to grow," said Bloomberg, one of the nation's leading pro-gun control advocates.

The investigation did not check those looking to sells guns and was limited to Armslist.com. It could not determine how many, if any, of the prospective felon buyers actually purchased weapons.

Bloomberg said the findings, which come in the wake of the Washington Navy Yard shooting that left 12 dead, were further evidence that Congress needed to pass legislation requiring background checks for all gun sales, including private sales where no such check is presently required.

"We know what happened this week has happened before — and sadly it looks like it will happen again until we get serious about this issue of just too many guns around," Bloomberg said in his first remarks about the Washington shooting.

A request for comment from ArmsList.com was not immediately returned. The site has disclaimers urging users to follow the law and to report illegal activity.

Some gun rights groups were angered by Bloomberg's surveillance of the online forum, which is largely used by legal buyers and sellers.

"I think that's creepy and disturbing," said Erich Pratt, director of communications for Gun Owners of America. "He's a petty little tyrant who wants to ban big sugary drinks and restrict the types of guns people can use to defend themselves."

Pratt also rejected Bloomberg's calls for more gun regulations.

"What needs to happen is less regulation," he said. "We need to end gun-free zones at schools and military bases that make people sitting ducks."

The Navy Yard shooter, Aaron Alexis, passed a background check at a Virginia gun shop before he bought the shotgun he used in his rampage.