Mironov evidently was displeased with his superior and other KGB Officers working in the KGB Rezidentura, and denounced Zarubin and others in anonymous letters to both Josef Stalin and FBI Director J. Edgar Hoover. Mironov had followed Zarubin to some of his clandestine meetings with American agents and in his letter to Stalin specified the dates and hours of these meetings, alleging that Zarubin was contacting the FBI. The letter caused Zarubin's recall to Moscow. The investigation against him and his wife, Elizabeth Zarubina, lasted six months and established that all his contacts were legitimate and valuable, and that Zarubin was not working with the FBI.
In August 1943, the anonymous letter drafted on a Russian typewriter was mailed in Washington. The original text in Russian was addressed to "Mr. Guver." It identifies Soviet "intelligence officers and operations that stretched from Canada to Mexico." This extraordinary note denounced Zarubin and 10 other KGB officers in North America, along with two of their assets, Hollywood film producer Boris Moros and Earl Browder, CPUSA General Secretary. FBI Special Agents quickly concluded that the letter was genuine and largely accurate, although they gave little credence to its claim of passing secrets to Japan.
The FBI subsequently increased surveillance of persons named in the letter and were able to get Moros and Andrey Shevchenko who worked at Bell Aircraft Niagra Falls to cooperate as double agents. Nevertheless, the FBI did apparently not pass copies of the anonymous letter to other agencies until after World War II, nor did Special Agents try to recruit the Soviet officers identified by its author. It also includes accusations of war crimes against the KGB Rezident in Washington, Vassili M. Zarubin (a.k.a. Zubilin), and his deputy, Markov (in the United States under the alias of Lt. Col. Vassili D. Mironov).
The letter asserted that Zarubin and Mironov were directly implicated in the bloody occupation of eastern Poland during the Nazi-Soviet alliance of 1939-41 and the murder of some 15,000 Polish soldiers—officers and NCOs, regulars and reservists—captured by the Red Army. The letter provided accurate and early confirmation of Soviet complicity in the executions in the Katyn Forest were German occupation forces in April 1943 discovered a mass grave containing 4,300 Polish corpses. Only someone "in the know" could have revealed that Polish soldiers had been interned at Kozelsk and Starobelsk and that Polish soldiers had been killed "near Smolensk." This information was known to only a handful of people in 1943 and was carefully concealed for almost 50 years by Soviet authorities.
Mironov was recalled from Washington and arrested on charges of slander, but when he was put on trial, it was discovered that he was schizophrenic. He was hospitalized and discharged from the service. Mironov was hospitalized in a Special Security hospital in Leningrad. In July 1945, pending to the newly established circumstances indicating high treason and continuous talk about top secret matters among unauthorized persons, Mironov was condemned to death by firing squad.
The KGB officers named in the letter are,
- Vasili Zarubin, Northa Aemrican Rezident;
- Elizabeth Zarubina, Zarubin's wife;
- Pavel Klarin, New York vice-consul;
- Gregory Kheifets. San Francisco Rezident;
- Leonid Kvasnikov, New York Deputy for X-line, technical intelligence;
- Semyon Semenov, New York X-line;
- Andrey Shevchenko, KGB agent operating as an inspector at Bell Aircraft Niagra Falls
- Sergey Lukianov, Soviet Government Purchasing Commission, Zarubin's assistant for naval intelligence;
- Vladimir Pavlov, 2nd Secretary of the Soviet Legation in Ottawa;
- Lev Vasilevsky (Leon Tarasov) Mexico City Illegal Rezident;
- Vasily Dolgov, 2nd Secretary, Washington, DC rezidentura;
Semyon Semenov in New York and Gregori Kheifetz in San Francisco were also identified in the letter. Regarding Semenov, the letter said, "SEMENOV works in AMTORG, is robbing the whole of the war industry in America. SEMENOV has his agents in all the industrial towns of the U.S.A., in all aviation and chemical war factories and in big industries. He works very brazenly and roughly, it would be very easy to follow him up and catch him red handed." Pavel Sudoplatov, head of the NKVD's Administration for Special Tasks wrote in 1992 that the author of this letter is Mironov.
"'Mr. Guver': Anonymous Soviet Letter to the FBI." , undated [received 7 August 1943] 
- Pavel Sudoplatov, with Anatoli Sudoplatov, Jerrold L. and Leona P. Schecter, Special Tasks: Memoirs of an Unwanted Witness and Soviet Spymaster (New York: Little, Brown, 1994), pp. 196-197.
- US House of Representatives, Committee on Un-American Activities, "Soviet Espionage Activities in Connection With Jet Propulsion and Aircraft," 81st Congress, 1st Session, 1949, pp. 101-128.
- Robert Louis Benson, and Michael Warner, eds. VENONA: Soviet Espionage and the American Response, 1939-1957, pg. 9. Washington, DC: National Security Agency/Central Intelligence Agency, 1996. Laguna Hills, CA: Aegean Park Press, 1996.
- Document No. 10 in Robert Louis Benson and Michael Warner, eds., Venona: Soviet Espionage and the American Response, 1939-1957 (Washington, DC: National Security Agency/Central Intelligence Agency, 1996).