A city investigation caught online gun sellers around the country illegally handing over assault weapons to people who said they couldn't pass a background check, Mayor Michael Bloomberg said Wednesday, just days after a police officer was shot to death with an illegal gun.

Peddlers who list their guns on websites such as Craigslist aren't required to conduct a background check before selling a gun, but it's against federal law for them to sell to someone they believe wouldn't pass such a check. Among the weapons purchased in the sting was a 9 mm semi-automatic handgun identical to the model police say was used to kill Officer Peter Figoski when he responded to a 911 call about a home invasion Monday.

The $290,000 effort _ the latest in a series of city-sponsored sting operations targeting out-of-state illegal gun sales _ took private investigators hired by the city far beyond New York's borders. But Bloomberg said Wednesday that the impact of such illegal sales makes them very much the city's business. The city estimates that 85 percent of guns used in crimes here come from out of state.

"Peter Figoski was our police officer. We didn't overstep anything," the mayor said at a City Hall news conference. "We obeyed the federal laws. We were very careful. ... We did not want to jeopardize any future prosecution."

Bloomberg said the city had submitted the results of its inquiry to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, which didn't immediately return a message seeking comment Wednesday. The private investigators used Craigslist and other websites to contact 125 online gun sellers in 14 states. Of those, 77 agreed to sell to buyers who said they couldn't pass a background check. The investigators followed through and completed five of the purchases in-person, at locations in Ohio.

The mayor called on the websites to institute checks to discourage illegal sales _ for example, by requiring sellers and buyers to provide identification. Craigslist doesn't officially allow the listing of guns on its pages, but the investigation found that gun sellers were openly using its pages and frequently flouting the law.

Zachariah Terhark, the owner of gunlistings.org, said his site asks users to report illegal activity to the ATF and removes postings that have been flagged by users. When people post on the site, Terhark said his company captures their IP addresses, which it can turn over to police if questions arise.

"Because gunlistings.org serves all 50 states, there many different sets of state and local laws that apply to firearm transactions. Due to this, we place the responsibility of knowing and following the gun laws on our users. Our buyers and sellers agree to this when posting a firearm or contacting a seller," he said.

Craigslist and other sites targeted in the investigation, including Glocktalk.com and KSL.com, didn't immediately return messages seeking comment.

Bloomberg, who leads a national coalition of mayors advocating stronger gun control, also renewed his call for Congress to close what he said were loopholes allowing buyers with criminal records or mental illnesses to buy guns.

The National Rifle Association, which has been opposed to such legislation, didn't return a call seeking comment Wednesday.

Figoski, a 22-year New York Police Department veteran, was killed early Monday during a botched armed robbery of a marijuana dealer living in a basement apartment in Brooklyn, police said. Five men, including Lamont Pride, are accused of plotting the robbery. They are being held without bail on murder charges.

The crew smashed in the door and began beating the dealer, and the upstairs owner of the home called 911 to report a break-in. Figoski and his partner were providing backup to two officers questioning the victim and two suspects inside the apartment when Pride and another man tried to flee, police said.

Figoski, 47, was shot once in the face as Pride tried to escape, before he had time to draw his weapon, police said. Pride's attorney urged people not to rush to judgment.

"It's going to be a long process. We're gathering information and urging everyone to wait to judge until the court system takes it course," James Koenig said.

Pride had been arrested twice in recent months in New York for weapons and drug possessions but released after police found they had no grounds to hold him under North Carolina warrants for his arrest, police said. The initial warrants were in-state only, meaning they didn't include extradition.

North Carolina officials eventually amended the warrants to require Pride be held until state authorities could retrieve him, but by then he had been released pending a January court date in New York, New York Police Commissioner Raymond Kelly said.

A spokeswoman for the Greensboro Police Department, Susan Danielsen, said that "the initial warrants for Mr. Pride were in-state only because we had no indicators that he was a flight risk."

The gun Pride is accused of using in the shooting this week was purchased legally in Virginia, but the former owner said it disappeared in 2009 when he moved. A second gun, a revolver found stashed inside the microwave at the crime scene, was shipped from Smith & Wesson in 1966 to an Indianapolis dealer now out of business.

Figoski's career included more than 200 arrests and 12 medals _ one of them an exceptional merit award for coming under fire in a brush with a man who would later be convicted as the city's Zodiac copycat killer of the early 1990s.

A funeral was set for Monday.


Samantha Gross can be reached at www.twitter.com/samanthagross