The fire erupted June 26 and has since scorched 189 square miles, or about 121,000 acres, of land northwest of Santa Fe. It is 19 percent contained.
Until Sunday, officials hadn't released a cause of the fire, which initially caused alarm as it burned close to the Los Alamos Nuclear Laboratory.
Los Alamos and nearby communities were evacuated nearly a week ago, but about 12,000 residents were allowed to go home Sunday.
Fire crews were still working Sunday to stop the blaze from burning too far through a canyon on the Santa Clara Pueblo reservation and threatening other pueblos on the Pajarito Plateau.