A Cambridge-educated physicist who had been identified as a spy code-named ALEK in papers purloined from the Soviet embassy in Ottawa in September 1945 by the defector Igor Gouzenko, May was placed under surveillance by MI5. An unsuccessful attempt was made to entrap him with an MI5 agent, Klop Ustinov, masquerading as his Soviet contact. May failed to attend the rendezvous in London compromised by Gouzenko and he was arrested. Confronted with the evidence, he was sentenced to 10 years’ imprisonment in 1946. He had been a Communist since he was 16 and was completely uncooperative with his interrogators, refusing to name anyone else involved in the Canadian spy ring.
May had arrived in Montreal in 1943 to work on the Manhattan Project, and in January the following year had visited to the MetLab in Chicago, accompanied by a dozen other scientists. He returned in April for a fortnight and was back again at the Argonne Laboratory at the end of August for a three-day conference to discuss the Argonne pile and the planned Montreal pile. His final trip to the United States took place at the end of September 1944, but a proposed stay of a month in the spring of 1945 was vetoed by the Manhattan Project’s security chief Gen. Leslie Groves on the grounds that it was contrary to policy for a single individual to learn too much from different areas of research.
After his release from prison, May returned to live in Cambridge, where he died in 2003. On his deathbed, he made a detailed confession to his family, but never disclosed when he had been recruited as a spy or by whom.