“When you see a four-inch deck gun, you know you’re not dealing with a fishing boat,” said Ryan King, a member of the technical dive team that discovered the ship.

The Eagle 56 sank on April 23, 1945, off the coast of Cape Elizabeth while towing a practice target for bombers from the nearby Naval Air Station Brunswick. The ship was part of a class of submarine chasers built by the Ford Motor Co. to combat German U-boats in World War I, but saw their first substantial action in World War II.

The German submarine U-853 destroyed the Eagle 56, and less than two weeks later the Navy sank the submarine near Block Island, R.I. The attack on the Eagle 56 occurred less than a month before Germany surrendered to the Allies. It took about 15 minutes for the warship to sink to the ocean floor after a 300-foot water column shot through the air, said Paul M. Lawton, a naval historian and lawyer from Warren, Mass., who first pursued the cold case of the Eagle.

“There were known to be almost a dozen U-boats in American waters in the final days of the war,” he said.

Mr. Lawton said he had developed an interest in the ship’s disappearance in the 1990s after two firefighters from Brockton, Mass., mentioned that their father had died aboard the Eagle.