Oxley was 4 nm inside Triton's area due to a navigational error. Lt.Cdr Steel of Triton made 2 challenges which went unanswered then fired a green signal grenade which also produced no response. He therefore had to assume the sub was hostile and fired 2 torpedoes, sinking Oxley.

The puzzling lack of response from Oxley was explained by Lt.Cdr Bowerman, who survived the sinking with one of the bridge watch: he stated he was called to the bridge following the sighting of Triton's signal grenade, but when he tried to answer, his grenade malfunctioned.
The OOD, Lt Manley, claimed to have answered Triton's challenge, but Bowerman was not sure this had been done properly. Before he could put things right, however, Oxley was struck by Triton's torpedo and he was catapulted into the sea.