Firefighters doused trees along a road near Las Vegas, N.M., earlier in May. Two New Mexico fires that merged into the largest wildfire in state history were each sparked by planned burns set by U.S. forest managers, federal investigators said. U.S. Forest Service fire investigators said Friday that a prescribed burn set in January to clear out vegetation in the Santa Fe National Forest turned into a sleeper fire that appeared dormant. It smoldered undetected through the winter and re-emerged in early April. Roughly 10 days later, high winds whipped the blaze , dubbed the Calf Canyon fire, beyond the control of firefighters, investigators said.