FARGO - A man who was sentenced to two life prison terms for killing a Wells County couple is now facing weapons charges in federal court.

Aron Nichols, 29, of Fargo, is charged with three felony counts, two for possession of illegal firearms and one for possession of unregistered firearms. He's accused of having a short-barreled shotgun and a machine gun.

U.S. Attorney Drew Wrigley said Nichols was indicted by a grand jury last fall, but federal authorities waited until after the murder trial to arrest him. The case was made public on Friday.

"It's kind of a procedural matter," Wrigley said. "We didn't want to serve him until the state procedures were complete, and now we're moving forward."

Nichols has not yet appeared in federal court.

Nichols and his fiancee, Tamara Sorenson, 30, were convicted in May for the deaths of Donald Willey, 70, and his wife, Alice, 67. Their bodies were found in their burned farm home near Sykeston in April 2007.

Sorenson was the mother of the Willeys' 8-year-old granddaughter, and authorities said there was a dispute about visitation rights to the child.

Prosecutors said Nichols shot the Willeys before setting their home on fire. He was sentenced to life in prison without parole for each of the killings. Sorenson received life in prison, but with the possibility of parole.

Nichols originally was charged in Cass County for possession of the shotgun, which authorities said was found during a search of his home in Fargo. That charge was dropped in anticipation of the federal case.

If Nichols is convicted in federal court, any punishment would be tacked on to the state sentence, Wrigley said.

"That may sound strange for someone who's sentenced to life in prison without parole," Wrigley said. "But you don't want to get into a situation where you're thinking, 'That's enough sentence over there.' There's always appeals or other matters that could come up."