It was one of the most shameful episodes in the history of the Allied war effort. Basically a German U-Boat torpedoed an armed troopship, not realizing it contained hundreds of POW's. It then pulled to and picked up survivors as was normal nautical practise but upon seeing the vast numbers of survivors the U-Boat radio'd for assistance to any vessel, promising not to attack regardless of the nationality of the would be rescuer. If memory serves, several more U-Boats arrived to help but the Allies would noty respond, thinking it was a trick. Then a US Liberator flew over and despite the U-Boats flying red crosses launched a bombing run, killing many survivors and forcing the U-Boats to cut the remaining lifeboats adrift.
As a result of this action the Germans ordered that from that point on no ship was to stop to pick up foreign survivors, an order that certainly cost the lives of thousands. Donitz gave the order and it was part of the reason he was convicted of war crimes despite the Americans following the same practice in the Pacific theatre. All in all a shameful part of the war.