Updated

LOS ALAMOS, N.M. -- Firefighters held their ground against the largest wildfire ever in New Mexico as officials at the nation's premier nuclear weapons laboratory and in the surrounding city planned for the return of thousands of evacuated employees and residents.

The blaze was several miles upslope Saturday from Los Alamos National Laboratory, boosting confidence that it no longer posed an immediate threat to the facility.

Thousands of experiments, including those on two supercomputers and studies on extending the life of 1960s-era nuclear bombs, have been put on hold because of the fire. Hundreds of employees began returning to the lab Saturday to begin the process of getting things ready for scientists, technicians and other employees to return to work.

Employees were checking filters in air handling systems to ensure they weren't affected by smoke from the fire, utilities were operational, as well as restarting computer systems shutdown when the lab closed.

"Once we start operation phases for the laboratory, it will take about two days to bring everyone back and have the laboratory fully operations," Lab Director Charles McMillan said. "I'd like to continue to ask the employees of the laboratory to continue to be patient."

Authorities didn't give a timetable for when they would lift evacuation order that began Monday for the town of Los Alamos, home to 12,000 people.

But some county workers were already back to prepare for the eventual rush of utility service calls, as well as possible flooding from surrounding mountainsides denuded by the wildfire. Los Alamos County Fire Chief Doug Tucker sent some of his firefighters home to rest in anticipation of having a fire department capable of responding to regular calls from residents returning home.

Fire officials were sure the fire wouldn't spread into the lab along its north and west boundaries. While they are confident the fire won't spread down Los Alamos Canyon and into the town and parts of the lab, firefighters were planning on burning out grasses and shrubs along the western edge of Los Alamos even though that area burned in 2000.

That the fire is burning in areas west of Los Alamos and in Santa Clara Canyon, north of town, that the Native American tribe there holds sacred, is an indication of how severe the fire danger is in the southwest. Burn areas typically provide spots to help stop fires.

The fire has blackened more than 177 square miles in the last six days, making it the largest in New Mexico history. Erratic winds and dry fuels helped it surpass a 2003 fire that took five months to burn through 94,000 acres in the Gila National Forest.

A key challenge Saturday continues to be stopping the flames from doing more damage to the lands of Santa Clara Pueblo. The fire had made a run north toward the reservation earlier this week, hitting the pueblo's watershed and cultural sites.

Santa Clara wasn't the only Indian community feeling the effects of the fire. To the south, Cochiti Pueblo was also worried about damage to ground cover affecting its watershed.

Also, the Pajarito Plateau has hundreds of archaeological sites at Bandelier National Monument that hold great significance to area tribes. About half of the park has burned, Bandelier superintendent Jason Lott said.

"The impact to our pueblos is unprecedented," said U.S. Rep. Ben Ray Lujan.

More than 1,200 firefighters were on the lines trying to slow down the flames as National Guard troops, state police officers and deputies patrolled neighborhoods and enforced evacuation orders.

Fire operations section chief Jerome Macdonald said parts of the fire in Santa Clara canyon burned hot while other areas saw less damage because of overnight temperatures and lighter winds.

In Los Alamos, fire officials said that crews worked to keep flames from spreading down a canyon that leads to the lab and the town. Los Alamos Fire Chief Doug Tucker said a small fire lit to remove fuels was steadily burning and being monitored by 200 firefighters.

The canyon runs past the old Manhattan Project site in town and a 1940s-era dump site where workers are near the end of a cleanup project of low-level radioactive waste, as well as the site of a nuclear reactor that was demolished in 2003.

Most of the town's displaced residents have been staying with friends or family. The American Red Cross has set up two shelters where 110 people have been staying.

Evacuees at the shelter at the Santa Claran Hotel Casino in Espanola, about 20 miles from Los Alamos, said the first night was the most difficult because of the commotion of people settling in and getting used to sleeping in a room with dozens of strangers.