On August 7, 1943, the director of the FBI received an anonymous letter written in Russian. It purported to name leading KGB officers operating under diplomatic cover in Soviet offices in the United States, Canada, and Mexico and charged that they were engaged in espionage on a broad scale. The letter stated that the chief KGB officer in the United States was Vasily Zubilin, that Zubilin's real name was Zarubin, and that his wife, Elizabeth, was also a KGB field officer running her own network of American sources. Other KGB officers named in the letter were Pavel Klarin and Semyon Semenov, officials at the Soviet consulate in New York; Vasily Dolgov and Vasily Mironov, officials at the Soviet embassy in Washington; Grigory Kheifets, Soviet vice-consul in San Francisco; Leonid Kvasnikov, an engineer with Amtorg; Andrey Shevchenko and Sergey Lukianov, officials with the Soviet Government Purchasing Commission; Vladimir Pavlov, second secretary of the Soviet embassy in Canada; and Lev Tarasov, a diplomat at the Soviet embassy in Mexico.

The FBI was, not surprisingly, perplexed by the letter and suspicious that it was a fraud. But an investigation of the activities of the Soviet diplomatic personnel named in the letter quickly convinced the bureau that they probably were indeed Soviet intelligence officers. Years later, the deciphered Venona messages further confirmed the accuracy of the identifications provided in the letter.

The motive behind the letter was clear: the anonymous author hated Vasily Zubilin and accused him of a variety of sins, including participating in the murder of thousands of Polish prisoners of war in the Katyn forest. This last accusation caught the attention of American authorities because at that time they were not sure what had happened at Katyn, and out of nowhere came a letter asserting inside knowledge about one of the participants in the Katyn action. Only a few months earlier, the German government had announced that it had uncovered a mass grave containing the bodies of thousands of executed Polish military officers in the Katyn forest near Smolensk, on Soviet territory overrun by Nazi forces. According to the Nazis, the Soviet Union had captured these Poles in 1939 when it conquered eastern Poland under the terms of the Nazi-Soviet Pact. The USSR blamed the mass murder on the Nazis, saying that the Germans had captured the Poles alive when they overran Soviet prisoner-of-war camps and had subsequently murdered them. In fact, the Soviets had murdered the Poles: on March 5, 1940, Stalin ordered the KGB to shoot 14,700 Polish prisoners of war.

The anonymous letter also correctly asserted that Zubilin had some role in the KGB's Katyn operation. The FBI had no way to verify it at the time, but eventually the Venona Project deciphered a KGB cable in which Zubilin himself confirmed having played a role. On July 1, 1943, he reported to Moscow that he thought he had noticed surveillance of his activities by a hostile intelligence agency and speculated that it had found out about his 1940 service at one of the camps at which the Poles had been murdered.

But while the claim that Zubilin had taken part in the Katyn massacre was accurate, the letter also contained the outlandish claim that he had betrayed the Soviet Union and was spying on the United States in the service of Japan. It urged American authorities to reveal Zubilin's treachery to Soviet authorities and asserted that when his betrayal was revealed, one of the other KGB officers, Vasily Mironov, would surely execute Zubilin on the spot. Mironov, nominally a Soviet diplomat, was described as a patriotic KGB colonel who hated Zubilin.

The FBI suspected that the author of the anonymous letter was a disgruntled KGB officer, but it was never sure of his identity. A passage in the 1994 memoir of a retired KGB general, Pavel Sudoplatov, suggests that Mironov wrote the letter. Sudoplatov, who held a headquarters role in KGB foreign intelligence operations during World War II, states that Mironov, a KGB lieutenant colonel, had sent a letter to Stalin denouncing Zarubin (the anonymous letter was correct about Zubilin's real name) as a double agent.

Mironov's letter caused Zarubin's recall to Moscow. The investigation against him and Elizabeth lasted six months and established that all his contacts were legitimate and valuable, and that he was not working with the FBI. 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.

Zarubin was secretly denounced to the FBI by Vasili Mironov, a senior officer in the New York residency who had earlier appealed unsuccessfully to the Centre for Zarubin's recall. In an extraordinary anonymous letter to Hoover on 7 August 1943, Mironov identified Zarubin and ten other leading members of residencies operating under diplomatic cover in the United States, himself included, as Soviet intelligence officers. He also revealed that Browder was closely involved with Soviet espionage and identified the Hollywood producer Boris Morros (FROST) as a Soviet agent. Mironov's motives derived partly from personal loathing for Zarubin himself. He told Hoover, speaking of himself in the third person, that Zarubin and Mironov "both hate each other". Mironov also appears to have been tortured by a sense of guilt for his part in the NKVD's massacre of the Polish officer corps in 1940.

Zarubin, he told Hoover, "interrogated and shot Poles in Kozelsk, Mironov in Starobelsk". (In reality, though Zarubin did interrogate some of the Polish officers, he does not appear to have been directly involved in their execution.) But there are also clear signs in Mironov's letter, if not of mental illness, at least of the paranoid mind set generated by the Terror. He accused Zarubin of being a Japanese agent and his wife of working for Germany, and concluded bizarrely: "If you prove to Mironov that Z is working for the Germans and Japanese, he will immediately shoot him without a trial, as he too holds a very high post in the NKVD.

By the time Mironov's extraordinary denunciation reached the FBl, Zarubin had moved from New York to become resident in Washington - a move probably prompted by the steady growth in intelligence of all kinds from within the Roosevelt administration. As the senior NKVD officer in the United States, Zarubin retained overall control in Washington of the work of the New York and San Francisco residencies; responsibility for liaison with the head of the CPUSA, Browder, and with the head of the illegal residency, Akhmerov; and direct control of some of his favourite agents, among them the French politician Pierre Cot and the British intelligence officer Cedric Belfrage, whom he took over from Golos.