Authorities have charged a woman for allegedly providing a convicted killer with the Bushmaster AR-15 assault rifle he used when he ambushed four volunteer firefighters and an off-duty cop at a fire scene in upstate New York on Christmas Eve, federal prosecutors said.

Dawn Nguyen, 24, was arrested today after allegedly making an illegal purchase of the weapon used by William Spengler, 62, who set a house and car on fire in Webster, N.Y., the morning of Dec. 24, then shot dead two firemen and himself.

Nguyen is facing federal and state charges for acting as a "straw purchaser," buying the Bushmaster assault weapon as well as a shotgun with the intention of giving it to someone who cannot legally purchase it himself, said U.S. Attorney William J. Hochul.