The Japanese submarine RO - 33 sank the 850 - cubic - metre ( 300 grt ) MV Mamutu by gunfire on 6 August 1942 .
Machine gunned survivors in the water .
The Japanese submarine RO - 33 sank the 850 - cubic - metre ( 300 grt ) MV Mamutu by gunfire on 6 August 1942 , and machine gunned survivors in the water .