Companies responsible for cleaning up a decommissioned plutonium plant in rural Washington state failed to conduct comprehensive safety checks at facilities containing nuclear waste, even after a 2017 tunnel collapse put surrounding communities on lockdown, government auditors reported Thursday.

The report about the Hanford nuclear waste site raises new concerns about environmental and safety risks posed by one of the United States’ worst toxic waste sites.

The Government Accountability Office found that the Energy Department waived a “root cause analysis” of the tunnel collapse because it was asked to do so by the contractor handling inspections, a subsidiary of Dallas-based Jacobs Engineering. The department did conduct a separate review to determine weaknesses and risks related to contaminated facilities, but that evaluation “was based largely on old data” and “did not include any physical or non-physical inspection” to flag facilities for cleanup, the office reported.

Sitting in a rural area of southwestern Washington, the Hanford site was once the U.S. military’s primary source of enriched plutonium used in nuclear warheads, including one of the weapons dropped on Japan at the end of World War II. Hanford’s workforce once numbered more than 50,000 people. Plutonium production ended in 1987.

Parts of the site have not been entered or inspected in more than 50 years, the Government Accountability Office reported, suggesting there could be additional safety risks of which the Energy Department is not aware. And the inspections that were carried out found structural problems severe enough that they “could lead to the potential release of hazardous or nuclear materials” at five of 18 facilities there, the office reported.

Energy Department spokesman Geoff Tyree referred reporters to statements provided in the office’s report. Jacobs Engineering did not respond to a request for comment.

David Trimble, the director of the accountability office’s natural resources and environment team, said the Energy Department needs a better approach to handling safety inspections at Hanford.

“The cleanup of the weapons complex is a huge undertaking with many facilities not expected to be cleaned up for many decades,” Trimble said in an email Friday. “Given the scope and nature of this contamination, it is critical for DOE to have an effective program to identify and mitigate any risks before they threaten workers, nearby communities or the environment.”

Since the late 1980s, the Energy Department has worked with teams of contractors on the monumental task of dealing with radioactive waste that accumulated over several decades. The massive scale and longevity of the weapons production activities at Hanford mean cleanup efforts are likely to continue for most of the next century.

The project has been fraught with waste, with milestones continually pushed back as contractors experienced difficulties. Earlier reports found that the department spent more than $19 billion over 25 years on “treatment and disposition of 56 million gallons of hazardous waste” without actually treating any hazardous waste. The project was originally scheduled to be completed in 2011 at a cost of $4.3 billion.

Besides the cost overruns, the haphazard way in which some waste was stored has made cleanup a hazardous task for the thousands of workers.

Industrial equipment used to process plutonium was stowed away in underground tunnels held up by wooden timbers. Although studies conducted in 1978 and 1991 found the tunnels structurally sound, the agency failed to conduct a recommended follow-up study that might have recognized structural problems with the tunnels, the Government Accountability Office included in its report.

After a partial collapse of one of the tunnels in 2017, the Energy Department declined to perform a “root cause analysis” that might have shed light on what caused the cave-in, a decision the office said was made at the request of the contractor.

“According to a written explanation provided to us by [the contractor’s] management, while the tunnel collapse was due to structural degradation, [the contractor’s] first priority was stabilizing the tunnel to mitigate the potential for further collapse, and a programmatic root cause analysis to determine the cause was not warranted,” the office wrote in its report.

Instead, the agency carried out an “apparent cause analysis,” which determined that the tunnel probably collapsed because the timber holding it up had decayed.

Other parts of the Hanford site have gone without safety inspections of any sort, including some used to store nuclear waste, the office wrote. Some of those areas, including processing facilities known as “canyons” where plutonium was extracted from uranium fuel rods, haven’t been entered or inspected in more than 50 years, the office wrote.

U.S. Sen. Ron Wyden (D-Ore.) scolded the Energy Department for its handling of the nuclear waste cleanup effort in a letter to Energy Secretary Dan Brouillette. The letter notes that the department has accepted all of the office’s recommendations but says those changes are not sufficient to protect the lives of workers and citizens throughout the region.

Wyden blamed the 2017 tunnel collapse on the Energy Department’s failure to conduct comprehensive inspections.

The tunnel collapse “seems largely due to a failure of [the Energy Department] and its contractors to independently verify the tunnel’s physical condition ― a state of affairs replicated over many years across the site’s facilities,” Wyden wrote.