Counter-terrorism authorities are investigating after a bulk shipment of unregistered—and potentially lethal—nuclear material was ferried off of an airplane at Heathrow Airport’s Terminal 4 on Dec. 29. The cargo, which originated in Pakistan and was smuggled into London from Oman aboard the hold of a passenger flight, was intended for Iranian nationals in the U.K., according to The Sun. “The race is on to trace everyone involved,” said an unidentified Heathrow source who spoke with the newspaper. “Security bosses are treating this with the seriousness it deserves. Protocol was not followed and this is now an anti-terror operation. There are real concerns over what the Iranians living here wanted with non-disclosed nuclear material,” they added. The shipment has triggered fears that a dirty bomb, a mixture made of explosives and radioactive material, was being built in Britain. No arrests have yet been made in the case.