USN Records of Crewmen lost at sea.

I was wondering if anyone might be able to help with this question. Whilst researching my Family Tree we have identified a relation who was a crewman on the SS Santa Lucia. She was taken into service by the USN as the USS Leedstown which was sunk shortly afterwards after being damaged by a torpedo from German submarine U-331 and later bombed and sunk by German aircraft off Algiers with the loss of about a third of her crew.
We know that he was naturalised as an American Citizen during the handover period and believe that he was kept on board the ship as a member of the crew.

What we are trying to find out is if he was one of the casualties. Can I ask if anyone knows where the USN keep their records of casualtied so we can follow up on this. Any help would be appreciated.

Thanks