(CNN) -- More than 2,100 firefighters were battling a massive wildfire Monday that was threatening hundreds of buildings and ancient sites on Indian lands in northern New Mexico.
The Las Conchas fire has scorched more than 121,000 acres and destroyed more than 100 buildings, including 63 homes, the federal Incident Management Team reported early Monday on its website. The fire was 19% contained, it said.
"We are not out of the woods yet," said Brad Pitassi, an Incident Management Team spokesman for the Southwest. "This will take a long time to contain."
Weather conditions Monday called for isolated rain showers and thunderstorms by early afternoon with a high near 92 degrees, according to the National Weather Service. Calm winds were expected to be about 5 and 10 mph, the service said.
Despite improving weather conditions that were allowing firefighters to begin to get an upper hand, the federal incident team website said the fire was threatening 410 structures early Monday.
Los Alamos residents can go home RELATED TOPICS
Federal agencies also warned that heavy smoke and ash was making air quality unhealthy as far away as Albuquerque, Santa Fe and Taos, and warned the elderly, children and those with heart or lung problems to stay indoors, according to a news release from federal agencies associated with the firefight.
Still, for all the continued challenges, firefighters made some significant progress over the weekend.
Most notably, Los Alamos County Police Chief Wayne Torpy announced it was lifting a majority of roadblocks around the county, allowing roughly 10,000 residents to return home.
The evacuees' return, Pitassi said, was largely due to progress subduing flames on the southwest edge of Los Alamos. But a lot of work remains to be done, with firefighters viewing battling the blaze -- like many others like it around the region -- as "marathons, not sprints."
"We've been really encouraged the last 48 hours," Pitassi said.
Investigators determined the fire, which began June 26, was sparked after an aspen tree knocked down by strong winds struck a power line and caught fire, said New Mexico State Forester Tony Delfin. The tree then hit the ground and sparked nearby vegetation, he said.