Visitors to the Grand Canyon may have been contaminated with unsafe radiation, according to officials.
A host of unmentioned buckets of dangerous uranium ore had been sitting in a research facility for decades, as people visited the building.
The five-gallon buckets have now been taken out of the building that is home to the park's archives and artefacts, according to National Park Service spokeswoman Vanessa Lacayo, and disposed of safely.
But they had been left there for nearly 20 years and meant the roughly 600 people who tour the facility each year had been exposed to them.
Ms Lacayo said the agency is working with Arizona health and workplace safety officials on the investigation.
The Arizona Republic cited the Grand Canyon's safety director in saying the park did not warn workers or the public of the potential harm that existed for years.
Uranium can be harmful depending on the amount, how people interact with it and the exposure time.
Additional reporting by agencies
