British anti-terror police are investigating after a package contaminated with traces of uranium was detected during routine checks at London’s Heathrow airport.

A “very small amount” of contaminated material was identified during the scan of a package that arrived at the hub on Dec. 29. Border Force officers then contacted the Counter Terrorism Command of the Metropolitan Police, according to the force.

The material has been assessed by experts “as posing no threat to the public,” and inquiries so far suggest that it’s not linked to any direct threat, Met Commander Richard Smith said in a statement.

No arrests have been made and officers are working with partner agencies on a full investigation, the Met said.

UK media reports said the package had arrived on a passenger flight from Oman. Heathrow declined to comment, saying the issue is a “police matter.”