J. Edgar Hoover
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Hoover expanded the FBI into a larger crime-fighting agency and instituted a number of modernizations to policing technology, such as a centralized fingerprint file and forensic laboratories. Hoover also established and expanded a national blacklist, referred to as the FBI Index or Index List.
Later in life and after his death, Hoover became a controversial figure as evidence of his secretive abuses of power began to surface. He was also found to have routinely violated both the FBI's own policies and the very laws which the FBI was charged with enforcing, to have used the FBI to harass and sabotage political dissidents, and to have extensively collected information on officials and private citizens using illegal surveillance, wiretapping, and burglaries. Hoover consequently amassed a great deal of power and was able to intimidate and threaten high-ranking political figures. Provided by Wikipedia