A recent leak at a US underground nuclear waste dump in New Mexico has resulted in 13 workers testing positive for radiation exposure, officials have announced.

The accident is the first-known release of radiation since the dump began taking plutonium-contaminated waste from US nuclear bomb building sites 15 years ago.

The US Department of Energy and the contractor that runs the Waste Isolation Pilot Project (WIPP) declined to comment further on the preliminary test results announced on 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.

All employees who were working at the plant when the leak occurred late on February 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.

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 on Monday before a community meeting in Carlsbad.

The leak 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.

WIPP is the first deep underground nuclear repository in the US 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.