The Finnish authorities on Wednesday impounded 160 tons of explosives and 69 surface-to-air Patriot missiles that were found on a British-flagged cargo ship. While the ship’s ultimate destination was Shanghai, officials said, it was not clear where the weapons were bound.

The cargo ship, M/S Thor Liberty, sailed from the German port of Emden on Dec. 13 and two days later docked in Kotka, Finland, to pick up a cargo of anchor chains, said a Finnish Customs spokesman, Petri Lounatmaa.

Investigating officers did not know the origin of the missiles or who was supposed to receive them, Mr. Lounatmaa said.

“We have impounded the explosives and missiles and asked the Defense Ministry to transport and store them,” he said. At this point, he said, it had not been determined “if the Thor Liberty planned a drop before its port of destination in China.”

Detective Superintendent Timo Virtanen of the National Bureau of Investigation said dock workers found the explosives stored on open pallets instead of in closed containers. They alerted inspectors, who found the missiles in containers that were marked as fireworks.

Interior Minister Paivi Rasanen said she had not heard of a similar case. “Of course, there are legal transports of weapons or defense material, but in this case the cargo was marked as containing fireworks,” Ms. Rasanen told the national broadcaster YLE TV. “That is quite unusual.”