Communist spies working for Stalin held positions which allowed them access to confidential information, and positions which gave them some measure of influence in decision-making. American army intelligence, however, kept much of this secret. To communicate it to upper-level members of FDR's administration would be to risk further security leaks: the KGB would discover that the Americans had broken their codes.
Eventually, non-military branches of the government learned independently of the security breaches. Whittaker Chambers, a former Soviet agent, revealed information about the spy network to a member of FDR's administration. Historian Ann Coulter writes:
A friend of Chambers had arranged a private audience with President Roosevelt's assistant secretary of state, Adolf Berle. After dinner at Berle's home, Chambers spent several hours detailing the Communist espionage network of which he had been a part. He gave Berle the names of at least two dozen Soviet spies, working for the Roosevelt administration. Among them was Alger Hiss, a top State Department official, as well as his brother Donald Hiss. Berle urgently reported to President Roosevelt what Chambers had said, including the warning about Hiss. The president laughed.
Roosevelt either didn't believe that the Soviets could do this, or he felt that any Communist infiltration was slight and benign and not worth worrying. Perhaps he thought Stalin wouldn't do it. Historians have long debated the extent to which Stalin was able to deceive Roosevelt. FDR was certainly aware that Stalin was deceptive, but was Roosevelt aware of the full extent of Stalin's willingness to mislead?
As it turns out, Alger Hiss was indeed a Soviet agent, sending secret information to Moscow, and receiving payment from the KGB. During Roosevelt's administration,
no action was ever taken against Hiss. To the contrary, Roosevelt promoted Hiss to the position of trusted aide who would go on to advise him at Yalta. Chambers's shocking and detailed reckoning of Soviet agents in high government positions eventually made its way to William C. Bullitt, former ambassador to Russia and confidant of the president. Alarmed, Bullitt brought the news to Roosevelt's attention. He, too, was laughed off.
Finally, after FDR's death in 1945, Hiss was convicted, confessed, and sent to prison.