Flooding in a tank that held clams caused a fishing vessel to capsize and sink off the coast of Massachusetts, according to an investigation by the National Transportation Safety Board (NTSB).

Two crew members were trapped on board and died when the uninspected fishing vessel Misty Blue sank on December 4, 2017. The other two crew members managed to escape and were rescued by a nearby fishing vessel. Oil sheens were observed on the surface of the water.

The crew had been harvesting clams nine miles southeast of Nantucket when the vessel began taking on water.

NTSB investigators determined that the probable cause of the incident flooding of the port clam tank from an undetermined point of ingress, which led to a decreased freeboard and a list allowing boarding seas to be trapped on deck, thereby decreasing the vessel’s stability. Contributing to the sinking was the relatively small freeing port area of the vessel, which likely increased water accumulation on deck.