Experts are monitoring America's only deep geological nuclear waste dump after an airborne radiation alert.
The Department of Energy said specialists were assessing a "possible radiological event" at the Waste Isolation Pilot Plant in Carlsbad, New Mexico.
According to the DOE, an air monitor detected radiation on the plant's underground levels at bout 11.30pm local time on Friday.
Officials say no employees were working underground at the time and those on the surface have been sheltered in place as a precaution. They say nobody has been found to be contaminated.
WIPP is the nation's first and only deep geological nuclear waste dump, taking plutonium-contaminated material from Los Alamos National Laboratory and other government nuclear projects.
The incident comes 10 days after an underground truck fire at the plant prompted an evacuation.
Energy Department spokesman Roger Nelson said the 139 workers above ground were told to stay where they were as a precaution. None of them tested positive for contamination and all non-essential staff were released, he said.
Mr Nelson said the cause of the leak was not yet known.