Authorities have found no signs of James “Randy” Udall, the missing brother of U.S. Sen. Mark Udall, D-Colo., after four days of searching a Wyoming mountain range where Randy went missing during a solo backpacking trip.

“Mark is concerned about his brother’s whereabouts, and he continues to closely monitor the situation,” Udall spokesman James Owens told the Washington Post on Tuesday. “He and his family hope for the best.”

Randy Udall, 61, is an energy consultant in western Colorado and a resident of Carbondale, Colo. He is an experienced backpacker, authorities told the Denver Post. Randy Udall took a trip last week to the Wind River Range -- a mountain range he has backpacked in before -- in Wyoming and has not been heard from since.

Federal and state authorities, including the U.S. Forest Service, have been searching for Randy Udall since Friday. A search was conducted Friday by Tip Top Search and Rescue of Pinedale, Wyo., a volunteer organization, the Post reported. Randy Udall officially went missing June 24, when he was expected to be back home, according to the Sublette County Sheriff’s Office.

"All the information we have is he had the supplies to be able to do his trip. As far as emergency beacons, we're not aware of any," Sublette County Sheriff Dave Lankford told ABC 7 News in Denver. About 225 square miles of the Wind River Range is being searched in hopes of finding Randy Udall.

"This is a large-scale search over a large area. We're working on elevations from 8,500 feet to 13,000," the sheriff said.

Mark Udall learned his brother went missing last week.

"In that time, [Sen. Udall] has remained in contact with those searching for his brother and, along with his family, has waited and hoped for the best,” Owens told the Post. "Nonetheless, he knows he still has a duty to lead -- even with his brother missing -- and has continued to serve while closely following the search."

Randy Udall was described as 6 feet 4 inches tall, 200 pounds with gray/brown hair and blue eyes. Anyone who believes they may have seen the missing man was asked to contact the Sublette County Sheriff’s Office at 307-367-4378.