The key figure behind the establishment of the Federal Bureau of Investigation (FBI) in the U.S. was J. Edgar Hoover. Appointed as the first Director in 1935, Hoover played a crucial role in transforming the FBI into a modern investigative agency. He implemented strict professional standards, emphasized the use of scientific methods in crime-solving, and expanded the Bureau’s responsibilities. Under Hoover’s leadership, the FBI focused on tackling organized crime, political corruption, and sustaining national security during tumultuous times, thus leaving a significant legacy in American law enforcement and civil rights.