Thousands of guns are being offered for sale illegally by unlicensed firearms dealers on a single website in the United States, according to a report released on Thursday by Mayors Against Illegal Guns.
The group, founded and largely funded by New York Mayor Michael Bloomberg, examined sales on Armslist.com, a major gun-trading website that enables sales in all 50 states, over an eight-week period this year.
Nearly a third of the ads posted were from "high-volume" sellers who lacked the federal license required of individuals who regularly sell firearms.
Private sellers are generally allowed to legally sell a gun through an online advertisement without checking the buyer's criminal record and mental-health history, although the rules governing such sales differ from state to state. Anyone engaged "in the business" of selling guns is required by federal law to have a license and to perform background checks on buyers.
The mayors group, based on its sample, estimated that nearly 240,000 guns would be advertised on the website in a year by high-volume sellers posing as private sellers who will not necessarily require a background check.
"These findings tell us that law enforcement, legislators and web sites all need to take steps to choke off this potentially deadly stream of illegal firearms sales," Bloomberg said in a statement.
He noted that the report was being released just two days before the nation marks the anniversary of the tragedy at a Connecticut elementary school, where a gunman killed 20 children and six adults.
"Nothing has changed in Washington since Newtown," Bloomberg said.
The report makes the latest example of Mayors Against Illegal Guns (MAIG) taking up an investigation to demonstrate what it sees as the failings of U.S. guns laws. In September, the group said it found that as many as 25,000 guns a year may be sold on Armslist.com to people with criminal records.
The group has long advocated for a private-sale loophole to be eliminated. Opponents of tighter gun-control laws say such efforts breach the Second Amendment of the U.S. Constitution.
Armslist.com offers a disclaimer that it "does not become involved in any transactions between parties" and its users must agree that they will not break the law. The website's operators did not immediately respond to a request for comment.
The site was created in 2009 by "gun-loving Americans after seeing first hand how the popular marketplace sites on the Internet shun firearms."
Bloomberg, one of the country's most prominent and wealthy advocates of tighter gun laws, steps down as mayor at the end of the year, but has said he will remain active on the issue.