University of York Russian specialist Shane O'Rourke states that the decossackization "was one of the main factors which led to the disappearance of the Cossacks as a nation". The late Alexander Nikolaevich Yakovlev, head of the Presidential Committee for the Rehabilitation of Victims of Political Repression, notes that "hundreds of thousands of Cossacks were killed". Historian Robert Gellately claims that "the most reliable estimates indicate that between 300,000 and 500,000 were killed or deported in 1919–20" out of a population of around three million.