Published: Aug 12, 2010 10:26 PM EDT
Updated: Aug 12, 2010 7:26 PM EDT

HELENA, Mont. (AP) - The trail has gone cold in the search for an escaped Arizona inmate and his suspected accomplice, with
baffled federal authorities saying they have no idea where the couple is and warning travelers stopping at campgrounds and truck stops to be alert.

John McCluskey and Casslyn Welch, two of the most wanted fugitives in America, are unpredictable and have long-haul truck
driving experience, meaning they are used to driving long distances over short time periods, Fidencio Rivera, chief deputy U.S. Marshal for Arizona, said Thursday. Their route has crossed more than 1,900 miles from their July 30 escape from a medium security prison in Arizona to the last concrete sighting in Billings, Mont., last Friday

"At this point in time, because they move so quickly and easily, we have no idea," Rivera said. "In two-and-a-half days
they could have traversed the entire United States. They can be anywhere."

Marshals are asking travelers at truck stops along highways and in campgrounds across the nation to watch out for the couple, who may have dyed their hair and otherwise changed their appearance. They may be sleeping in campgrounds and using truck stops to clean up as they travel in a 1997 Nissan Sentra that has been described separately as gray, gold and tan, Rivera said.

Authorities believe McCluskey, 45, and Welch, his 44-year-old cousin and fiancee, are dangerous and have been paying for their getaway by robbing businesses along the way. They and fellow escaped convict Tracy Province, who split from the group and was arrested in Wyoming on Sunday, also are suspects in the deaths of an Oklahoma couple whose burned bodies were discovered last week in eastern New Mexico.

"We're concerned that these individuals will commit more crimes," Rivera said. "They're desperate. These guys are not
thinking rationally."