The cargo ship Niel Maersk was built by Odense Staalskibsværft A/S in Odense, Denmark in 1931, in July 1941 the ship was seized at Baltimore and turned over to the War Shipping Administration. In August 1941 the ship placed under the management of the Isthmian Steamship Company and renamed Johnstown.

On June 5, 1942 the ship was sailing alone returning to the USA from Karachi and was en route to make a stop at Lourenço Marques, Mozambique when the Japanese submarine I-20, who had just made a successful attack on British ships a Diego Suarez, torpedoed and sank the ship in the Mozambique Channel.

I have not been able to locate details of the sinking, or what happened to the survivors (there were about 37), sources even differ on the number of casualties, most say 2, while others 4. The list I have shows six, 2 Merchant Mariners and 4 U.S. Navy gunners.

I-20 was one of the submarines which launched midget subs during the attack on Pearl Harbor and it was a midget attack at Diego Suarez which damaged HMS Ramillies and sank the tanker British Loyalty. In September 1943 I-20 failed to return from a sortie off Espiritu Santo, she was last heard from on Aug. 31. It is presumed she was sunk by a U.S. destroyer attack in early September.