London ? Rough seas swept four American sailors from the deck of a U.S. nuclear submarine off the coast of southwestern England on Friday, killing two of them, authorities said.

The USS Minneapolis-St. Paul was leaving Plymouth harbor accompanied by police boats in rough weather, police said. The four sailors were tied to the vessel with ropes – a routine precaution – but were knocked into the water by surging waves.

The police boats – small inflatable dinghies with outboard motors – picked up all four, said Baxter Provan, a spokesman for the Devon and Cornwall police.