For almost 20 years, workers and visiting schoolchildren at a Grand Canyon museum may have been unknowingly exposed to radiation from three buckets of uranium sitting next to a taxidermy exhibit, according to allegations from a National Park Service safety director.

The whistleblower says officials learned about the buckets last year and tried to hide the revelation, according to the Arizona Republic newspaper. This month, Elston “Swede” Stephenson emailed all park staff and brought the matter to the attention of the head of the interior department, which oversees the park service, and the agency’s internal watchdog.

The interior department told CNN that a recent survey showed radiation at “background” levels, which it said are always in the environment because uranium naturally occurs in the rocks of the Grand Canyon.

“There is no current risk to the public or park employees,” the department said.

Stephenson said the buckets were next to an exhibit of what appear to be taxidermied birds and rodents, where children on tours stopped for presentations, and he claims they would have received doses of radiation that exceed federal standards.

As Stephenson recounted to the Republic, the uranium was discovered by the teenage son of a park employee who had a Geiger counter. The buckets were moved to another location, and Stephenson called a national park specialist in Colorado, who sent technicians with a Ludlum meter, another tool for measuring radiation. He says the technicians removed the buckets wearing dishwashing and gardening gloves. And he claims they hid the radiation readings from him and dumped the ore into Orphan Mine, an old uranium mine nearby.

Inspectors later tested the empty buckets, which had been returned to the building.

In a statement, the park service confirmed that uranium rocks in three five-gallon buckets found in the building were moved to the Orphan Mine in June 2018. It added that the building is open by appointment only, and sees up to 1,000 visitors and researchers annually.

The Republic and CNN cited experts who suggested that uranium ore alone wouldn’t be dangerous unless inhaled or swallowed. But Edwin Lyman, the director of the nuclear safety project at the Union of Concerned Scientists, told the Guardian that it could over time decay into isotopes that pose more of a threat.

Lyman said if the dose measurements were accurate, people in close proximity would have been exposed to elevated levels of radiation. But he added that the dose levels dropped sharply with distance.

“Unless you were hugging the canister or within a meter or less for an extended period of time, then the dose by itself wouldn’t be a major concern,” Lyman said. “But people should try to have an idea of radiation they’re exposed to … because the lifetime dose is cumulative and your cancer risk is going to be related to that lifetime cumulative dose.”

The park service is coordinating an investigation with the Occupational Safety and Health Administration and the Arizona department of health services, according to the Republic.

Stephenson was also a whistleblower in the US navy, when he drew attention to falls following a series of accidents, the Republic said. He later issued a formal racial discrimination complaint against the park service, the newspaper said.

Describing the uranium concerns, he told the paper: “I’ve never seen anything like this in my life.”