US investigators were taking samples at a New Mexico underground nuclear waste site where airborne radiation was detected, though authorities stressed they had found no contamination.
Officials monitoring the possible radiation leak said there was no danger to people or the environment at the Department of Energy's Waste Isolation Pilot Plant, the nation's first repository to seal away radioactive waste, mostly plutonium, used for defense research and the production of nuclear weapons.
The waste is dumped 2,150 feet (655 meters) underground in disposal rooms excavated in an ancient, stable salt formation.
No employees were underground when the continuous air monitor at WIPP near Carlsbad, New Mexico detected underground radiation at 11:30 pm Friday (0630 GMT Saturday), an agency statement said.
It said no staffer was found to be contaminated by the radiation.
Employees on the surface were told to shelter in place as a precautionary measure and were cleared to leave the site starting at 5:00 pm Saturday (0001 GMT Sunday).
"We are continuing to monitor and we are emphasizing that there is no danger to human health and the environment," WIPP spokeswoman Deb Gill told AFP.
The agency stressed that "no contamination has been found on any equipment, personnel or facilities."
As soon as the airborne radiation was detected underground at the site WIPP's ventilation system automatically switched to filtration mode in order to prevent air exchange with the surface.
Investigators have not yet identified the source of the radiation, but WIPP said the site's system of air monitors and protective filtration system "continue to function as designed."
The site was shut down and not performing active operations at the time, according to Gill.
Earlier this month, an underground blaze prompted the evacuation of a different part of the site, after a truck hauling salt caught fire. Several workers suffered smoke inhalation.
But officials said the blaze was nowhere near radioactive material.
Material dumped at the site includes plutonium-contaminated waste from the Los Alamos National Laboratory, about 300 miles (500 kilometers) away, also in New Mexico.