Authorities expanded their search for Colorado ultrarunner Micah True on Friday after the 58-year-old went missing near Gila, N.M., in the state's southwest.

True, from Boulder, Colo., was last seen Tuesday morning at the Gila Wilderness Lodge when he went for a run, the Boulder Daily Camera reported. The lodge's owners reported him missing Wednesday morning after he failed to return.

Incident commander Frankie Benoist told the newspaper that 14 search and rescue teams continued searching for True on Friday, assisted by tracking dogs, horse teams, a National Guard helicopter and a fixed-wing aircraft.

True was the subject of Chris McDougall's 2009 best-seller "Born to Run," which chronicled his time running in Mexico's Copper Canyon with a local tribe known for their grueling running feats.

McDougall joined the search parties Friday.

"It's some intense wilderness, but I can't believe that so many people can't find him," he said. "We've gone over every place there is, and we can't figure out where he could possibly be."