California ? Strong downdrafts that overpowered the climbing ability of Steve Fossett’s airplane probably caused the wealthy adventurer to crash in mountainous terrain west of Mammoth Lakes, Calif., almost two years ago, federal accident investigators announced Thursday.

Fossett, 63, a record-setting balloonist and pilot, disappeared Sept. 3, 2007, after taking off for a pleasure flight from a ranch in Yerington, Nev. His single-engine plane failed to return, setting off an aerial search that lasted a month and combed 10,000 square miles.

Based on weather and wind conditions on the day of his disappearance, the National Transportation Safety Board concluded that Fossett probably encountered moderate turbulence with gusts up to 35 mph and downdrafts of at least 400 feet per minute, which forced him into the mountains.

Investigators said that downdrafts likely surpassed the plane’s maximum rate of climb of 300 feet per minute at an altitude of 13,000 feet and 370 feet at 12,000 feet. Fossett was flying in a part of the Sierras where the mountains exceed 13,000 feet in elevation.