ARDGLASS, Northern Ireland — After a day of fishing in the Irish Sea, Paul Murphy was about to head for home when his trawler, the Karen, suddenly shuddered to a halt.

A loud bang gave way to the sound of cables tensing. But when the Karen started moving again, it was being dragged backward, fast and at an angle.

“It was like the scene out of ‘Jaws’ when the boat took off — do you remember, the shark took the boat away?” said Mr. Murphy, the skipper, pointing to an electronic trace of the Karen’s unnatural, disjointed path that afternoon last month.

“But multiply it by 100,” he said. “It was just a bigger event.”

An 80-ton trawler that normally catches prawn in its nets, the Karen this time seemed to have ensnared a submarine. And, with the British Navy and NATO both denying involvement, suspicion has fallen on Russia, which since the conflict in Ukraine has been testing the response times of the alliance in the air and at sea.