The Department of Energy declared an emergency Tuesday at the Hanford nuclear site in Washington state following the collapse of a rail tunnel there.

Washington’s KING-TV reports that a portion of a tunnel that transports trains carrying radioactive material collapsed early on Tuesday morning, with officials warning workers at the facility to take emergency precautions.

A reporter for the station said the tunnel that collapsed is connected to the facility’s uranium extraction plant.

The Department of Energy declared an emergency at 8:26 a.m. local time, the agency said in a statement, after workers discovered a hole in the roof of the Cold War-era tunnel at the facility.

Officials evacuated some personnel close to the incident immediately while ordering others to shelter in place. By Tuesday afternoon, all employees near the site of the incident were released from their shelter-in-place order and told they could go home.

The agency said responders have not found any indication of contamination at the site, and they said the tunnel did contain contaminated materials.

First responders discovered soil subsiding above a tunnel connected to the extraction center during a routine inspection, the agency said.

In a statement, the Department of Energy said that “everyone has been accounted for and there is no initial indication of any worker exposure or an airborne radiological release.”

Washington Gov. Jay Inslee (D) called the incident a “serious situation” in a statement.

“Ensuring the safety of the workers and the community is the top priority,” he said.

“Federal, state and local officials are coordinating closely on the response, and the state Department of Ecology is in close communication with the U.S. Department of Energy Richland Office. My office is in close communication with these agencies and directly with Department of Energy headquarters in D.C. We will continue to monitor this situation and assist the federal government in its response.”

The Hanford site for decades produced plutonium for American nuclear weapons, including the bomb dropped on Nagasaki, Japan, in World War II.

The U.S. established Hanford in the 1940s, and today it is considered the most toxic place in the United States, with a local contractor undertaking a $110 billion clean-up of the 56 million gallons of chemical and nuclear waste still at the facility, NBC reported last year.