Both Mara and Bertram are innocent because if they had left Bertram Blount’s cabin, they would have been seen by Larita Brandenburg. Larita is innocent because Bertram Blount saw her leave Roscoe Barth’s cabin at 10:35pm, which was before the time of death. Mohamed Guess couldn’t have killed Roscoe because he was running to deck 8 at 10.45pm. Gertrude Weiss couldn’t have killed Roscoe because if she had, she would have been seen by Bertram Blount, therefore it had to have been Major Wren – the M.I.5 agent who was instructed to seize the nuclear material and dispose of the ‘spy’ who had it.
BUT HOW DID HE DID IT?
After leaving the bar, Major Wren returned to his cabin where he retrieved the heavy duty gloves and Geiger counter. Having served Roscoe Barth drinks, he was aware of Barth’s cabin number – 8010. Wren was occupying 9010, which is directly above Barth’s suite. Wren climbed down from his balcony to Barth’s balcony, picked the lock on the outer door and entered the suite. He then knocked out, gagged and tied Roscoe to the bed. While Roscoe was unconscious, Wren put on the gloves, broke into the suitcase and discovered that the material was radioactive waste. He then used it to burn Roscoe’s torso.