LONDON (AP) — British counterterrorism police are investigating after border officers seized cargo contaminated with uranium at London’s Heathrow Airport.

The Metropolitan Police force said Wednesday that “a very small amount of contaminated material was identified after routine screening within a package incoming to the U.K.” on Dec. 29.

It said the force’s Counterterrorism Command is investigating. There have been no arrests.

The BBC reported that the uranium was in a shipment of scrap metal. The Sun, which first reported the story, said the shipment originated in Pakistan and arrived on a flight from Oman.

Police Commander Richard Smith said the amount of contaminated material was “extremely small and has been assessed by experts as posing no threat to the public.”

He said it “does not appear to be linked to any direct threat,” but the investigation continues.

“We will of course, follow every avenue to see what the background to this was and satisfy ourselves that there’s no further threat,” he said.

Prime Minister Rishi Sunak’s spokesman declined to comment on an ongoing police operation.