They call him Caballo Blanco, the White Horse.
Ultrarunner Micah True, 60, is a legend in the distance running community. He was depicted in Christopher McDougall's best-selling book, "Born to Run," in which he took up residence in the remote Copper Canyons of Mexico among the Tarahumara, a desert-dwelling tribe of the best ultrarunners in the world who gave him the moniker.
But True mysteriously disappeared last Tuesday after leaving for what was supposed to be a 12-mile run in southwest New Mexico's Gila National Forest. Search efforts that were launched on Wednesday have ramped up across the region, The Associated Press reported.
Some 14 search teams -- complete with people on horseback, dogs, helicopters and a search plane -- set out Friday to find True, and they were joined by hordes of extra volunteers today, New Mexico state police told the AP.
"We're going to do everything possible to cover as much ground as possible, but it's already been four days," state police spokesman Lt. Robert McDonald told the AP. "By no means are we going to give up, but time is of the essence as always in a search and rescue effort."
True, who lives in Denver, had spent the night at the Wilderness Lodge in Gila Hot Springs, N.M., and went running Tuesday morning wearing shorts, a T-shirt and hat and carrying one water bottle, according to Jane Bruemmer, the lodge's co-owner.
"He goes for a run every day, so it's not unusual," she told ABCNews.com. "We just don't think he had much with him, since he was planning on coming home. We're perplexed."
Since True spends part of the year in Urique, Mexico, where he is the race director of the Copper Canyon Ultra Marathon, a 50-mile race across the desert, Bruemmer says he often stays at her lodge while en route to and from Mexico. He runs frequently among the cliff dwellings of the Gila National Forest.
Before True went missing, Bruemmer and her husband were heading to Silver City, N.M., a town about 40 miles away from the lodge, but True had planned to stay until Wednesday, when he was to depart for Phoenix to visit his girlfriend. So the couple bid him goodbye, leaving a friend to watch the lodge while they were away, Bruemmer said.
But the friend told the Bruemmers that True never returned from his run on Tuesday, and that "the guests were worried," she said. So she and her husband, Dean, called the state police Wednesday morning, and they launched a search in the area, where the temperature can drop below freezing at night.
True had left his dog at the lodge, since he would be running north along Highway 15, which dead-ends at the cliff dwellings, Bruemmer said.
"We don't know where he was from then on," she said. "We're assuming he went off-trail. But this is a huge wilderness."
Bruemmer said she's received lots of calls and emails from the ultrarunning community, offering their help with the search, and True's girlfriend also arrived from Phoenix to join the effort.
True wasn't known to have any health problems, and Bruemmer suspects he may have become injured. But the search parties "have gone over and over these trails, and on really obscure trails that are barely there anymore," she said.
People have been known to get lost in the Gila National Forest, because it's a wilderness area, "so you can get turned around," she said. "But I wouldn't have thought it would happen to Micah."