Operating out of Freemantle, Australia, the USS Sturgeon was on patrol west of the Philippines when she spotted an unescorted Japanese freighter. The patrol report completed by Lieutenant Commander W.L. Wright is one of the few concrete pieces of evidence known about the incident:
30 June 1942:
Patrolling northwest of Bojeador as before. Dove at dawn, surfaced at dusk. At 2216 sighted a darkened ship to southwest. At first, due to bearing on which sighted, believed him to be on northerly course, but after a few minutes observation it was evident he was on a westerly course, and going at high speed. He quite evidently had stood out of Babuyan Channel, headed for Hainan.
Put on all engines and worked up to full power, proceeding to westward in an attempt to get ahead of him. For an hour and a half we couldn’t make a nickel. This fellow was really going, making at least 17 knots, and probably a bit more, as he appeared to be zig-zagging. At this time it looked a bit hopeless, but determined to hang on in the hope he would slow or change course toward us. His range at this time was estimated at around 18,000 yards. Sure enough, about mid night he slowed to about 12 knots. After that it was easy.
1 July 1942:
Proceeding to intercept target as before. Altered course to gain position ahead of him, and dove at 0146. When he got in periscope range, it could be seen that he was larger than first believed, also that his course was a little to the left of west, leaving us some 5,000 yards off the track. Was able to close some 1,000 yards of this, and then turned to fire stern tubes as:
i) Only three tubes available forward, and at this range and with large target four torpedo spread desirable.
ii) After tubes had 70D/ heads, while heads forward were small ones.
At 0225 fired four torpedo spread, range 4,000 yards, from after tubes. At 0229 heard and observed explosion about 75-100 ft. abaft stack. At 0240 observed ship sink stern first. 0250 surfaced, proceeded to eastward, completing battery charge. Ship believed to be Rio de Janeiro Maru, or very similar type, although it is possible it was a larger ship, he was a big one.
A few lights were observed on deck just after the explosion, but there was apparently no power available, and his bow was well up in the air in six minutes. Dove at dawn, No further contacts.
Unfortunately Wright could not know that the ship was the Montevideo Maru and that she was packed with Australian Prisoners of War and Civilian Internees from New Britain. It is believed that 1056 Australain captives died plus over 70 Japanese naval personnel. It is now considered to be the worst maritime disaster to befall Australia.
Japanese silence over the incident was to exacerbate the disaster and continues to cause controversy to this day. The facts only became known at the end of the war, when a list of Australians captured at Rabaul was discovered. Only then did the Japanese admit they had been on the Montevideo Maru, whose loss had been recorded by the Japanese in 1942. Stories that some Japanese had escaped the sinking, even possibly a few of the Australian prisoners, have continued to circulate.
For more see Montevideo Maru Foundation