Evidence of Soviet espionage networks in the United States comes from different sources, which may be grouped into two categories: the data available during the Cold War, and the data which became available after it was over.
Some of this evidence was used to discover and convict Soviet agents during the Cold War. The Pumpkin Papers - actually, several rolls of film - were given by Whittaker Chambers to the United States government.
The Pumpkin Papers were documents which Chambers had couriered from their sources to Soviet operatives who would send them on to Moscow. Chambers had been a member of the Communist Party (CPUSA) during the early 1930s.
The CPUSA was not merely a group advocating policies and candidates. It was an arm of the Soviet government and functioned as part of a spy network. When Chambers left the party, he kept the Pumpkin Papers. Eventually, he aided the U.S. government and gave them the papers.
The papers received their odd name because Chambers had hidden them in a pumpkin.
Chambers produced the papers in 1948 and gave them to Congress. They were used in ongoing investigations and trials which ultimately revealed that Alger Hiss, a high-ranking State Department officer who personally advised the president on foreign policy, was a Soviet operative.
An editor of the University of Michigan’s Michigan Law Review writes:
Even before Soviet cables proved the existence of a vast Soviet-run espionage network in America, there was lots of evidence. There were, for example, the detailed accounts given in sworn testimony by various ex-Communists like Whittaker Chambers, Elizabeth Bentley, and Louis Budenz. There were Chambers's Pumpkin Papers. There were Soviet defectors who brought reams of KGB documents with them, identifying Soviet agents in America. There were confessions of arrested spies, such as David Greenglass, who informed on his sister Ethel Rosenberg and her husband, Julius. There was the arrest of Judith Coplon, who was actually apprehended in the act of handing a U.S. counterintelligence file to a KGB officer.
Among the data which became available after the end of the Cold War, the Venona Project files are probably the most famous. These were a series of messages between Soviet operatives which had been intercepted and decrypted by U.S. intelligence agencies.
Other evidence became available when the government of the Soviet Union, and the governments of other Warsaw Pact nations, fell after 1989. While the Venona Project had intercepted and decrypted telegraphed messages as early as 1942, the evidence was not released, even to other branches of the U.S. government, for fear that the Soviets would learn of the breach in their system.
Had the Soviet learned of the intercepts, the data would have become worthless, and the communists would have changed their encryption system. The intercepted messages made it clear that there were numerous Soviet operatives in various government agencies, so the data from Venona was not shared, even within the government.
It turned out that, all along, there was also evidence in the form of decrypted Soviet cables to their agents in America. Though not revealed for half a century, the U.S. government had broken the Soviet cable code beginning in the forties in a top-secret undertaking known as the Venona Project. In the most patriotic act of his career, Senator Daniel Patrick Moynihan would push through the declassification of the Venona Project, which was finally unveiled on July 11, 1995.
Keeping Venona secret for so long was a calculated risk: it meant that some Soviet agents would not be prosecuted. Those agents who were tried in court were convicted based on other evidence; even the prosecuting attorneys did not know about the Venona evidence.
After 1995, it became clear how large the Soviet espionage network in the United States was, and how thoroughly it had infiltrated the U.S. government.