Dec. 28, 2012 -- 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.

Authorities accuse Nguyen of lying about being the sole owner of the weapons when making the purchase, a violation of federal law.

As an ex-convict, Spengler could not have legally owned or purchased the weapons himself.

"Dawn Nguyen told the seller of these weapons that she was to be true owner and buyer of these guns," said Hochul. "It is absolutely against federal law to provide any materially false information" on a firearms application.

As Hochul announced the charges, Nguyen was in a nearby court. It was unknown whether she entered a plea, and her lawyer could not be immediately contacted.

In addition to the rifle and shotgun, Spengler was found with a pistol, which authorities believe he used to kill himself by shooting himself in the head.

After apparently setting the fires, Spengler began shooting at emergency responders, officials have said. The attack left two firefighters and the gunman dead, and two other firefighters hospitalized.

Police officer John Ritter recieved shrapnel injuries at the scene but was discharged quickly from the hospital.

In a typewritten note found at the scene, Spengler revealed that he obtained the weapons from Nguyen, who for a time lived next door to him in Webster, Hochul said.

In that same note, Spengler pledged to see "how much of the neighborhood I can burn down." He said he wanted to "do what I like doing best, killing people."

Police said Spengler set a "trap" in order to ambush the first responders.

Firefighters Michael Chiapperini and Tomasz Kaczowka were gunned down. Two other firefighters, Joseph Hofsetter and Theodore Scardino, remained in guarded condition at a Rochester, N.Y. hospital.