ALBUQUERQUE, N.M. >> Thirteen workers at the nation’s underground nuclear waste dump in southeastern New Mexico have tested positive for radiation exposure after a recent leak released toxic particles in and around the plant, officials announced.

The U.S. Department of Energy and the contractor that runs the Waste Isolation Pilot Project declined to comment further on the preliminary test results announced Wednesday, saying they’ll discuss the issue at a news conference Thursday afternoon.

“It is important to note that these are initial sample results,” the DOE and Nuclear Waste Partnership, the plant operator, said in a joint statement. “These employees, both federal and contractor, will be asked to provide additional samples in order to fully determine the extent of any exposure.”

All employees who were working at the southeastern New Mexico plant when the leak occurred late Feb. 14 were checked for contamination before being allowed to leave, the news release said. But biological samples were also taken to check for possible exposure from inhaling radioactive particles.

Elevated radiation levels have been detected in the air around the plant, but officials have said the readings are too low to constitute a public health threat.

The accident is the first-known release of radiation since the dump near Carlsbad began taking plutonium-contaminated waste from the nation’s nuclear bomb building sites 15 years ago. It came just nine days after a truck hauling salt in the plant’s deep mines caught fire, but officials say they are confident the incidents are unrelated.

Officials said they can tell from their analyses of air samples in and around the plant that a container of waste leaked, but it could be weeks before they can get underground to find out what caused it. Possible scenarios include a ceiling collapse or a forklift puncturing a canister, Farok Sharif, president of the Nuclear Waste Partnership, said Monday before a community meeting in Carlsbad.

More than 250 people attended that forum, where Sharif and Joe Franco, the DOE site office manager, told sometimes skeptical residents that the elevated amounts of radiation that have been detected offer no more risk than a dental X-ray or an airline flight.

Still, Sen. Tom Udall, D-N.M., said he will send the Environmental Protection Agency a letter Thursday requesting that they bring portable air monitors to the area.

“The health and safety of the Carlsbad community and WIPP personnel are my top priority,” Udall said.

New Mexico State University runs a monitoring center in Carlsbad that offers free radiation-detecting body scans. The director of the center said there has been a rise in appointments being scheduled since the leak.

WIPP is the nation’s first deep underground nuclear repository and the only facility in the country that can store plutonium-contaminated clothing and tools from Los Alamos National Laboratory and other federal nuclear sites.