The marshal for the District of Arizona, David Gonzales, said that Mr. Barksdale had “a history of involvement with white supremacy organizations,” and that there was evidence to suggest that the couple were getting help from such groups.

Mr. Barksdale, 56, is white, 6-foot-5 and has tattoos of swastikas, warlocks and lightning bolts, “all associated with white supremacy groups,” the marshals service said. It described Ms. Barksdale, 59, as “a 5-foot-7 white female.”

“I would stress to anybody that is assisting those individuals that we will come after them also,” Mr. Gonzales said.

The Tucson Police Department has charged the couple with first-degree murder, first-degree burglary, arson and other criminal offenses related to the death of Frank Bligh, 72, in April.

The authorities say that the couple, along with Mr. Barksdale’s nephew, Brent Mallard, 31, set fire to Mr. Bligh’s Tucson home and caused an explosion. Mr. Bligh’s body has not been found, but the authorities believe he was killed.