State and federal authorities are searching for Sen. Mark Udall's (D-Colo.) brother Randy, who went missing during a solo backpacking trip in Wyoming's Wind River Range.

Randy Udall, 61, is a respected renewable energy expert who lives in Carbondale, Colo., and regularly gives speeches about how to tackle climate change. He was expected to return from his trip last Wednesday.

Sublette County Sheriff spokesman Steve Smith told The Denver Post that authorities learned Friday that he was unaccounted for. Tip Top Search and Rescue of Pinedale, Wyo., coordinated a search that day that included ground teams and aerial sweeps with  helicopters. Teams continued looking over the weekend, with help from the U.S. Forest Service.

"Mark is concerned about his brother's whereabouts, and he continues to closely monitor the situation," said James Owens, a spokesman for the senator. "He and his family hope for the best."