The details of SS Scillin's loss and the circumstances of the death of the Allied POWs were kept secret for more than 50 years until persistent inquiries by relations and historians brought a more open response. The reasons for such official reticence for such a long period are not clear, but there are claims that it was deemed necessary to protect intelligence sources, notably ULTRA.

The lengths to which the Allies went to protect the knowledge of the existence of ULTRA were extraordinary, and cost many allied servicemen their lives, rather than let the Germans know that their communications were being read. 

In November 1942, in Tripoli, Libya, 814 Allied POWs were embarked in Scillin's hold, which, reportedly, was suitable for only about 300. The result was severe overcrowding and unsanitary conditions. More prisoners would have been embarked, but the British military doctor (Captain Gilbert, Royal Army Medical Corps) made vehement and repeated protests. Some reports state that a further 195 POWs were embarked before Scillin sailed and that there were some 200 Italian troops aboard; others dispute these points saying that the only Italian troops aboard were guards and gun crews and the surplus POWs were never actually embarked. She sailed on 13 November 1942. 

A  submarine, HMS Sahib, intercepted Scillin on the night of 14 November off the coast of Tunisia. Sahib first fired two shells at the cargo ship and then launched a single torpedo, which hit Scillin's hold and rapidly sank her. Prisoners in the hold had little chance of survival. Sahib rescued 27 POWs (26 British and one South African), Scillin's captain and 34 Italian crew and soldiers  before the arrival of an Italian warship obliged her to leave. Only when Sahib's crew heard survivors speaking English did they realise the ship's purpose

[DB/Various sources]