Jan. 29, 2020 will be the 75th anniversary of the largest loss of life in Coast Guard history, the explosion of USS Serpens (AK-97). We have already discussed this incident, but now there is a effort to look again at the cause of this loss.
Foxtrotalpha reports there may be reason to believe that the ship was torpedoed rather than having been destroyed by an ammunition loading accident.
I considered that it might have been an attack carried out by Kaiten, submarine launched manned suicide torpedoes. They were being used at that time to attack shipping in forward bases. Kaiten might have made an attack on a protected harbor easier, but the link in this paragraph provides a listing of operations that seems to preclude that possibility. That in spite of the fact that there were about 20 Kaiten capable Japanese submarines operational at the time of the sinking.