LOS ALAMOS - As crews fight to keep a New Mexico wildfire from reaching the nation's premier nuclear-weapons laboratory and the surrounding community, scientists are busy sampling the air for chemicals and radiological materials.

Smoke from the Las Conchas fire fills the sky near the Los Alamos Laboratory in Los Alamos, N.M., Tuesday, June 28, 2011. A vicious wildfire spread through the mountains above a northern New Mexico town on Tuesday, driving thousands of people from their homes as officials at the government nuclear laboratory tried to dispel concerns about the safety of sensitive materials. (AP Photo/Jae C. Hong) Their effort includes dozens of fixed-air monitors on the ground, as well as a "flying laboratory" dispatched by the U.S. Environmental Protection Agency. The special twin-engine plane is outfitted with sensors that can collect detailed samples.

Sen. Tom Udall of New Mexico requested the agency's help early on in the monitoring effort near the Los Alamos National Laboratory. EPA officials said the flying lab was set to make its initial data-collection fight Wednesday, and state and federal officials have vowed to make findings from all the monitoring efforts public.

"I know people are concerned about what's in the smoke," Udall said. He noted that the state, the Los Alamos lab and the EPA were all looking closely at air quality "so we can assure the public" there will be multiple layers of oversight.

The blaze had grown to more than 108 square miles by Wednesday morning, but firefighters managed to hold the line along the nuclear lab's southern boundary.

On its western edge, firefighters began targeted burns to rob fuel from the fire. Lab officials warned that people might see more smoke coming from the lab border, but they said there was no fire burning on the site as of mid-Wednesday.

Residents downwind have expressed concern about the potential of a radioactive smoke plume if the flames reach thousands of barrels of waste stored in above-ground tents at the lab.

Top lab officials and fire managers say there have been no releases of toxins. They say they're confident the flames won't reach key buildings or areas where radioactive waste is stored. As a last resort, foam could be sprayed on the barrels containing items that might have been contaminated through contact with radioactive materials to ensure they aren't damaged by fire, they said.

The site's manager for the National Nuclear Security Administration said he evaluated the precautions and felt comfortable. The agency oversees the lab for the Department of Energy.

"I have 170 people who validate their measures," Kevin Smith said. "They're in steel drums, on a concrete floor."

Despite the assurances, some residents remained concerned for the safety of their families and nearby communities.

"If it gets to this contamination, it's over _ not just for Los Alamos, but for Santa Fe and all of us in between," said Mai Ting, a resident who lives in the valley below the desert mesas that are home to the Los Alamos National Laboratory.

Chris Valvarde, a resident of the Santa Clara Pueblo about 10 miles north of Los Alamos, questioned officials at a briefing Tuesday evening, asking whether they had evacuation plans for his community. Los Alamos, a town of 11,000, already sits empty after its residents were evacuated ahead of the blaze, which started Sunday.

The wildfire has already sparked a spot fire at the lab. The fire Monday was quickly contained, and lab officials said no contamination was released.

Lab Director Charles McMillan said the barrels contain transuranic waste _ gloves, toolboxes, tools _ and other items that may have been contaminated. An anti-nuclear group had estimated there could be up to 30,000 55-gallon drums stored at a site known as Area G, but lab spokeswoman Heather Clark said Wednesday there are 10,000 drums stored there under fire-retardant tents.

Los Alamos County Fire Chief Doug Tucker, whose department is responsible for protecting the lab, said the barrels are stacked about three high inside the tents.

Area G holds drums of cleanup from Cold War-era waste that the lab sends away for storage in weekly shipments, according to lab officials.

Flames were just across the road from the southern edge of the famed lab, where scientists developed the first atomic bomb during World War II. The facility cut natural gas to some areas as a precaution. The lab will be closed through at least Thursday.

The streets of Los Alamos were empty again Wednesday, with the exception of emergency vehicles and National Guard Humvees. Homeowners who had left were prepared: Propane bottles were placed at the front of driveways and cars were left in the middle of parking lots, away from anything flammable.

The wildfire has destroyed 30 structures south and west of Los Alamos, for many stirring memories of a blaze in May 2000 that destroyed hundreds of homes and buildings in town.

Favorable winds have helped firefighters, who were busy trying to keep the fire from moving off Pajarito Mountain to the west of Los Alamos and into two narrow canyons that descend into the town and the lab.

The lab, which employs about 15,000 people, covers more than 36 square miles and includes about 2,000 buildings at nearly four dozen sites. They include research facilities, as well as waste disposal sites.

Some facilities, including the administration building, are in Los Alamos, while others are miles from the town. Most of the buildings from the Manhattan Project that developed the first atomic bomb in the 1940s were built on what is now the town and are long gone. The spot fire Monday scorched a section known as Tech Area 49, which was used in the early 1960s for a series of underground tests with high explosives and radioactive materials.

Lab spokesman Kevin Roark said environmental specialists were monitoring air quality, but the main concern was smoke. Lab personnel and the state environment department were monitoring the air for radioactivity and particulates.

Additional ground-based monitors were on their way, some of which would be placed in neighboring Espanola and Santa Fe.