Mayor Michael Bloomberg says undercover investigators working for New York City were not required to pass a background check at an Arizona gun show when they bought a pistol with an extended magazine, “like the weapon used in Tucson.”

Bloomberg says that the sale just days after the mass shooting in Tucson exposes a “dangerous gap” in federal gun laws.

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New York’s mayor also says the investigators were allowed to buy semi-automatic pistols from two other sellers after telling them they probably could not pass a background check. He says that under federal law, the sellers should have stopped the sale.

The investigators visited the Crossroads of the West Gun Show in Phoenix on Jan. 23.

The Jan. 8 shooting rampage in Tucson killed six people. It also wounded 13, including Rep. Gabrielle Giffords.