Note the mysterious death of Soviet defector Walter Krivitsky in DC. His widow and son would stay with Whittaker Chambers for several months.
Sudoplatov claimed to be responsible for Trotsky's assassination and was in charge of the Sooviet espionage effort to obtain the secrets of the atomic bomb from America and Great Britain. He set up a network of illegals who convinced Robert Oppenheimer, Enrico Fermi, Leo Szilard, Bruno Pontecorvoco, Alan Nunn May, Klaus Fuchs, and other scientists in America and Great Britain to share atomic secrets with the Soviets. He documents all sorts of dirty deeds and other than being quite out of character to not eliminate a defector, there does not appear to be any good reason to doubt him on this.
The circumstances of Krivitsky's demise are highly suspect, but there is a good chance that this might be the only such case for which they are not guilty. Maybe we can start a new conspiracy theory and claim that FDR ordered his elimination to get on the good side of Uncle Joe.