The infiltration starting in the FDR administration and continued apace afterwards. FDR was confronted with a list of names of Soviet spies and he just laughed at the FBI and told them to go away.
Some interesting names on the list including the father of Harold Ickes Jr. who was FDR’s advisor at Yalta. The Venona papers uncovered that link, but it wasn’t the only one.
One of FDR’s Vice Presidents visited Stalin’s gulags and said nothing.