In 2013 a lot of media coverage was given to unclassified NSA's study in Cryptologic Spectrum that concluded that Tito did not speak the language as a native, and had features of other Slavic languages (Russian and Polish). The hypothesis that "a non-Yugoslav, perhaps a Russian or a Pole" assumed Tito's identity was included. The report also notes Draža Mihailović's impressions of Tito's Russian origins.
What led everyone to decide Tito was not who he claimed to be?
NSA's study in Cryptologic Spectrum that concluded that Tito did not speak the language as a native