Search-and-rescue crews are combing the Wind River Range of Wyoming, looking for Randy Udall, the 61-year-old brother of Sen. Mark Udall, D-Colorado, who has not returned from a solo backpack trip.

“He is missing,” said Sublette County Sheriff’s spokesman Steve Smith. “He was supposed to be back on the 26th. We were notified on the 28th.”

Tip Top Search and Rescue of Pinedale, Wyo., coordinated a search Friday with resources that included ground teams and aerial sweeps with two helicopters, Smith said. The search continued over the weekend with assistance from the U.S. Forest Service.

Sen. Udall has known about the disappearance since last week, his office told The Denver Post.

“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,” spokesman James Owens said. “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.”

Udall held events Sunday, urging the U.S. House of Representatives to pass the Senate’s immigration reform package with Sen. Michael Bennet and former Interior Secretary Ken Salazar. On Monday, Udall held another immigration reform event at the Denver Hispanic Chamber of Commerce.

Randy Udall, who lives in Carbondale, is the second-oldest of six children. He is an experienced backpacker who has hiked in the Wind River Range many times over the past 30 years, authorities said.