A mixture that included organic cat litter forced the closure of the only underground nuclear waste repository in the US, a team of government experts has determined.
The investigation was launched after low-level radioactive material was released in February 2014.
It found an incompatible combination of nitrate salts and cat litter caused a breach in a container of waste.
The cat litter was used to absorb moisture in the waste.
More than 20 workers at the Waste Isolation Pilot Plant in New Mexico were contaminated and the release forced the repository to close indefinitely.
Investigators believe that a thermal reaction inside the container forced the lid to pop.
Federal officials have said it could take years and more than $0.5bn to fully reopen the plant.
"We are reviewing the report and its findings now as we continue working to mitigate the risks of this happening again," the Energy Department said in a statement issued on Thursday.