The next meeting of the Combined Policy Committee on 15 April 1946 produced no accord on collaboration, and resulted in an exchange of cables between Truman and Attlee. Truman cabled on 20 April that he did not see the communiqué he had signed as obligating the United States to assist Britain in designing, constructing and operating an atomic energy plant. Attlee's response on 6 June 1946 "did not mince words nor conceal his displeasure behind the nuances of diplomatic language." At issue was not just technical co-operation, which was fast disappearing, but the allocation of uranium ore. During the war this was of little concern, as Britain had not needed any ore, so all the production of the Congo mines and all the ore seized by the Alsos Mission had gone to the United States, but now it was also required by the British atomic project. Chadwick and Groves reached an agreement by which ore would be shared equally.The defection of Igor Gouzenko and the resulting espionage conviction of Alan Nunn May, a British physicist who had worked at the Montreal Laboratory, made it politically impossible for US officials to exchange information with the UK. The McMahon Act, which was signed by Truman on 1 August 1946, and went into effect at midnight on 1 January 1947, ended technical co-operation. Its control of "restricted data" prevented the United States' allies from receiving any information. The remaining scientists were denied access to papers that they had written just days before. The McMahon Act fuelled resentment from British scientists and officials alike, and led directly to the British decision in January 1947 to develop its own nuclear weapons. In the United States, there was a furore over the British veto over the use of nuclear weapons when the Joint Committee on Atomic Energy was informed of the Quebec Agreement (but not the November 1945 agreement) on 12 May 1947, resulting in intense pressure on Truman to drop the provision. On 7 January 1948, Bush, James Fisk, Cockcroft and Mackenzie concluded an agreement known as the modus vivendi, that allowed for limited sharing of technical information between the United States, Britain and Canada, which officially repealed the Quebec Agreement. Like the Quebec Agreement it replaced, the modus vivendi was classified "Top Secret".As the Cold War set in, enthusiasm in the United States for an alliance with Britain cooled as well. A September 1949 poll found that 72 per cent of Americans agreed that the United States should not "share our atomic energy secrets with England". The reputation of the British was further tarnished by the 1950 revelation that Fuchs was a Soviet atomic spy. British wartime participation in the Manhattan Project provided a substantial body of expertise that was crucial to the success of High Explosive Research, the United Kingdom's post-war nuclear weapons programme, although it was not without important gaps, such as in the field of plutonium metallurgy. The development of the independent British nuclear deterrent led to the McMahon Act being amended in 1958, and to a resumption of the nuclear Special Relationship between America and Britain under the 1958 US–UK Mutual Defence Agreement.

Question: What are the last names of the people whose actions caused the exchange of information between US officials and the UK impossible?
Answer: Gouzenko, May