MOAB, Utah, May 2 (UPI) -- A Colorado rock climber pinned by an 800-pound boulder for five days in a narrow Utah canyon cut part of his right arm off and then climbed down a cliff and walked to safety, officials said Friday. Aron Ralston, 27, of Aspen, was in serious condition at St. Mary's Hospital in Grand Junction, Colo., where he was questioned by the National Park Service about his solo trek near the Canyonlands National Park in southeast Utah. Ralston, an experienced climber and outdoorsman, told rescuers that he opened his pocketknife and cut off the forearm Thursday to save his life. He said he had run out of water two days earlier. Ralston put an tourniquet on his arm, administered first aid, and then eased himself down more than 70 feet to the bottom of Blue John Canyon. He then walked out until he ran into two hikers who escorted him to a rescue helicopter searching the area. Ralston had planned a one-day trip when he embarked Saturday, but it turned into a harrowing adventure. He apparently had only begun the hike when the boulder fell on him in the rugged, narrow canyon. Ralston was flown to a hospital in nearby Moab and then transferred late Thursday to St. Mary's Hospital where he underwent surgery. Ralston is a veteran mountain climber who has scaled many of the 14,000-foot peaks in Colorado. He escaped death only last February in an avalanche on Tennessee Pass in Colorado, according to the Denver Post. A National Park Service official expressed concern Friday that Ralston was climbing alone and apparently had not advised anyone of his plans. "Most truly experienced people don't break those rules, first hiking alone in that kind of situation where you know you are going to be doing technical climbing, and second, not alerting somebody, telling them I am going such and such a place," said Jim Blazik, an official at Canyonlands National Park. Blazik said the accident occurred in the park but Ralston was apparently in and out of the park boundary during his trek. A search had been started Thursday when someone reported that Ralston had not shown up for work in Aspen for four days, the Post reported. A search party returned to the canyon after Ralston's rescue to try to retrieve his arm but they could not remove it from under the boulder, Emery County Sheriff's Deputy Mitch Vetere told the Post on Thursday.