Historians have gathered data from multiple sources about the Soviet spy network. The United States Army was able, by means of its Operation Venona, to intercept and decode a number of encrypted messages between Moscow and communist operatives in America.
Other sources of information include files from various intelligence agencies: files which were declassified and made accessible to the public once the Cold War had ended.
But some of the biggest sources of evidence about Soviet intelligence-gathering inside the United States are the files and archives of the Federal Bureau of Investigation (FBI). Historians Stan Evans and Herbert Romerstein write:
Of importance also — though an underrated resource — are the confidential archives of the FBI, which was tracking and recording the activities of Communists and Soviet agents in the United States before Venona came on line and before the advent of the Cold War. In some recent studies the efforts of the FBI in this regard have been disparaged, but, on close inspection, these negative comments aren’t backed up by the record. In some cases of the New Deal years the Bureau may have missed clues it should have noted, but by the early 1940s it was far ahead of other U.S. agencies in spotting and combating the infiltration problem.
From these data, it is clear not only that the Soviet espionage network inside the United States was massive, and that it was highly effective at sending classified secrets back to the Kremlin, but also that it exerted a subtle but significant pressure on U.S. policymakers.
Not only did the Soviet have access to confidential information, but they also were influencing decisions made inside the U.S. government by means of well-placed secret agents.
Some of those agents, like Alger Hiss, worked at the highest levels of government: Hiss had face-to-face meetings with the president.
The President of the United States was taking advice from a communist spy!