(CNN) -- The brother of U.S. Senator Mark Udall of Colorado has gone missing after backpacking alone in the Rocky Mountains.
Randy Udall, 61, went on a hike of the Wind River Range in Wyoming on June 20 and was expected back six days later.
After the experienced hiker did not return, family members reported him missing, said the Sublette County Sheriff's office.
Rescue crews and helicopters scouring mountain passes have been unable to locate him, said sheriff's spokesman Stephen Smith.
Udall was familiar with the area, Smith said. The trail he was on was well marked, but the terrain was rough.
Sen. Mark Udall, a Democrat, was elected into office in 2008. Before that he served in the House of Representatives for 10 years.
He has been an outspoken critic of the collection of telephone and metadata by the National Security Agency and advocates changes to the Patriot Act to protect Americans' right to privacy.