Through the Golos spy network, Halperin provided Soviet intelligence with a large quantity of sensitive U.S. diplomatic dispatches, including reports from Ambassador John Gilbert Winant in London on the position of the Polish government-in-exile towards negotiations with Stalin, Turkey's foreign policy toward Romania, the State Department's instructions to the U.S.
Halperin also distorted OSS reports with false information in order to reflect the views of Stalin, the Soviet Union, and the Communist Party of the United States.
[1] On July 31, 1948, ex-Soviet spy Elizabeth Bentley testified under subpoena before the House Un-American Activities Committee and related details which she first shared with the FBI in 1945.
Bentley added that "some time early in 1945 'JACK', [Soviet agent Joseph Katz][3] the Russian contact at that time, told me that Halperin had been accused by General William J. Donovan, the head of OSS, of being a Soviet agent..."[4] The next day, the FBI notified Harry S. Truman's White House that "according to a "highly confidential source," among those "employed by the government of the United States" who "have been furnishing data and information to persons outside the Federal government, who are in turn transmitting this information to espionage agents of the Soviet government," was "Maurice Halperin, Office of Strategic Services."
Subsequent surveillance of Halperin disclosed that he was in contact with Nathan Gregory Silvermaster, Lauchlin Currie, Philip and Mary Jane Keeney, and others.
[citation needed] In 1953, after Soviet cables were secretly decrypted by U.S. counter-intelligence, Maurice Halperin was called before the Senate Internal Security Subcommittee to defend himself on charges of espionage, at which time he lost his teaching position at Boston University.
[citation needed] In Vancouver, he became a political science professor at Simon Fraser University, and wrote several books critical of Castro's government and the socio-political situation in Cuba.