The "gun show" loophole allows gun purchases without a background check
Undercover New York City investigators bought high-powered pistols from an Arizona gun fair without undergoing a background check, officials said.
One made the purchase despite telling the seller he was "probably" barred by law from gun ownership, a report said.
Mayor Michael Bloomberg's office said the investigation exposed "a dangerous gap in our existing federal gun laws".
The show was held two weeks after six were killed in a shooting at an Arizona congresswoman's constituent meeting.
Representative Gabrielle Giffords, a Democrat, is recovering from a gunshot wound to the head. Among the six killed were a nine-year-old girl and a federal judge.
Jared Loughner, 22, is being held pending trial for the attack.
According to a report released on Monday, a team of private investigators hired by New York City visited the Crossroads of the West Gun Show in Phoenix, Arizona on 23 January.
'No questions asked'
Without undergoing background checks to ensure they were not prohibited from gun ownership, the investigators purchased 9mm semi-automatic pistols, including a Glock similar to the weapon Mr Loughner is accused of using in the deadly 8 January attack.
"We have demonstrated how easy it is for anyone to buy a semiautomatic handgun and a high capacity magazine, no questions asked," Mr Bloomberg said.
Federal law prohibits convicted criminals, mentally ill individuals and drug abusers from owning guns.
Licensed firearm dealers are required to conduct an FBI instant background check on prospective buyers.
But a provision of the background check law - which gun control advocates term the "gun show loophole" - allows private citizens to sell weapons from their personal collections without a background check.
"Gun shows provide a central marketplace for prohibited purchasers to connect with private sellers who make anonymous gun sales," Mr Bloomberg's report stated.
And it is illegal for private parties to sell guns to individuals they "know" or "have reason to believe" are barred from gun ownership.
According to the report, an undercover investigator purchased pistol despite telling the seller "I probably couldn't pass" a background check.
Bob Templeton, the president of the Crossroads of the West show, told the New York Times: "When we find someone who isn't complying with the law, we ask them to leave or don't allow them back."