A company acted negligently in selling the weapon used in a Christmas Eve shooting that left two New York volunteer firefighters dead, according to a lawsuit filed Tuesday.

The semi-automatic rifle was used by William Spengler Jr. on Dec. 24, 2012, during a pre-dawn ambush in which he killed two West Webster firefighters and wounded two others. The gun was purchased at a Gander Mountain store by his neighbor, Dawn Nguyen. Spengler couldn’t buy the gun because he was a felon.

The Brady Center to Prevent Gun Violence says in the complaint filed on behalf of the four shooting victims that Gander Mountain should have realized Nguyen was making a “straw purchase” for Spengler. He had accompanied her into the store to buy multiple firearms with cash—indicators of a possible straw purchase, the suit says.

A spokesman for St. Paul, Minnesota-based Gander Mountain said the firearms were purchased with all the proper documentation and that the individual who bought the weapons “intentionally deceived” the company. “We will vigorously defend ourselves in this matter,” he said.

Nguyen and Spengler’s estate also are named in the lawsuit, which was filed in Monroe County State Supreme Court. In addition to the Brady Center, the plaintiffs are represented by Arnold & Porter in New York and Woods Oviatt Gilman in Rochester.

Spengler set fire to his home on Lake Ontario before ambushing the responders. He then killed himself. The remains of Spengler’s sister were found in the burned-out home. She had been shot in the head.

Nguyen, 25, was sentenced Monday to a maximum term of 11/3 to four years in state prison for lying on forms when she bought the guns. She faces additional federal charges.