Thousands of guns could be sold illegally by unlicensed firearms dealers on just one classified ad website, according to an investigation commissioned by New York City Mayor Michael Bloomberg.

Bloomberg, one of the nation's leading gun-control advocates and a co-founder of Mayors Against Illegal Guns, asked his investigators to spend eight weeks monitoring the ads on Armslist.com. The classified marketplace, known as a "Craigslist for guns," is one of the nation's largest firearms websites.

The inquiry found that nearly one-third of gun ads on the site were posted by high-volume sellers who do not possess the mandatory federal firearms license. At that pace, nearly 244,000 guns would be sold illegally a year due to the "private sale loophole," which does not require a background check, Bloomberg said Thursday.

"Unlicensed sellers of firearms are flooding the Internet with weapons," Bloomberg said at City Hall news conference. "The result is a massive online, largely unregulated, secondhand firearms market that threatens the safety of all of us."

A representative from ArmsList.com did not immediately return a request for comment.

The inquiry was a follow-up to one the gun-control group did earlier this year in which investigators secretly conducted background checks of would-be buyers and found that many of them had felony records that, by law, prohibit them from buying firearms.

Bloomberg noted that he was announcing the probe's results just days before the anniversary of the shooting at Sandy Hook Elementary School in Newtown, Connecticut, which killed 20 students and six adults.

Bloomberg is leaving office at month's end, but he vowed that he will continue to stay involved – and fund – the pro-gun control organization.

"We will keep fighting," he said. "Maybe we'll keep fighting even harder, we'll have a little more time to focus on this."

Mayor-elect Bill de Blasio has signaled his approval of Bloomberg's gun-control efforts but has not said what role he might play in the mayors' group.