the early thirties, there was an epidemic of bank robberies in the Midwest orchestrated by colorful criminal gangs who took advantage of superior firepower and fast get-away cars to bedevil local law enforcement agencies. To the chagrin and increasing discomfort of authorities, such robbers were often viewed as somewhat noble in their assaults upon the banking industry, which at the time was evicting many farmers from their homesteads. That empathy reached the point that many of these desperadoes, particularly the dashing John Dillinger (who became famous for leaping over bank cages and his repeated escapes from jails and police traps), were de facto folk heroes whose exploits frequently captured headlines. State officials began to implore Washington to aid them in containing this lawlessness. The fact that the robbers frequently took stolen cars across state lines (a federal offense) gave Hoover and his men the authority to pursue them. Things did not go as planned, however, and there were some embarrassing foul-ups on the part of the FBI, particularly clashes with the Dillinger gang (actually led by "Handsome" Harry Pierpont).
Hoover in 1935.A raid on a summer lodge named "Little Bohemia" in Manitowish Waters, Wisconsin, left an agent and a hapless civilian bystander dead, along with others wounded. All the gangsters escaped. Hoover realized that his job was now on the line, and he pulled out all stops to capture the culprits. Hoover was particularly fixated on eliminating Dillinger, whose misdeeds he considered to be insults aimed directly at him and "his" bureau. In late July 1934, Melvin Purvis, the Director of Operations in the Chicago office, received a tip on the whereabouts of John Dillinger. That paid off when the gangster was cut down in a hail of gunfire outside the Biograph Theater.
Because of several highly-publicized captures or shootings of outlaws and bank robbers including Dillinger, Alvin Karpis, and Machine Gun Kelly, the Bureau's powers were broadened and it was re-named the Federal Bureau of Investigation in 1935. In 1939, the FBI became pre-eminent in the field of domestic intelligence. Hoover made changes, such as expanding and combining fingerprint files in the Identification Division to compile the largest collection of fingerprints ever.
Hoover also helped to greatly expand the FBI's recruitment and create the FBI Laboratory, a division established in 1932 to examine evidence found by the FBI.
Hoover was concerned about subversion, and under his leadership, the FBI spied upon tens of thousands of suspected subversives and radicals. Hoover tended to exaggerate the dangers of subversives, and many times overstepped his bounds in his pursuit of eliminating that perceived threat.
The FBI had some successes against actual subversives and spies. For example, in the Quirin affair during World War II, German U-boats set two small groups of Nazi agents ashore in Florida and Long Island to cause acts of sabotage within the country. Although the members of these teams were apprehended, this was due entirely to the fact that one of the would-be saboteurs went to the FBI and confessed. Nevertheless, President Harry Truman wrote in his memoirs: "The country had reason to be proud of and have confidence in our security agencies. They had kept us almost totally free of sabotage and espionage during the World War II".
Another example of Hoover's concern over subversion is his handling of the Venona Project. The FBI inherited a pre-World War II joint project with the British to eavesdrop on Soviet spies in the UK and the United States. Hoover kept the intercepts — America's greatest counterintelligence secret — in a locked safe in his office, choosing not to inform Truman, his Attorney General McGraith, or two Secretaries of State — Dean Acheson and General George Marshall — while they held office. He informed the Central Intelligence Agency (CIA) of the Venona Project in 1952.
According to documents declassified in 2007, Hoover maintained a list of 12,000 Americans suspected of disloyalty with the intention of detaining them and to do so by suspending the writ of habeas corpus. Hoover submitted his plan to President Harry Truman at the outbreak of the Korean War, but there is no evidence that Truman accepted the plan.
 COINTELPRO years
In 1956, Hoover was becoming increasingly frustrated by Supreme Court decisions that limited the Justice Department's ability to prosecute Communists. At this time he formalized a covert "dirty tricks" program under the name COINTELPRO. This program remained in place until it was revealed to the public in 1971, and was the cause of some of the harshest criticism of Hoover and the FBI. COINTELPRO was first used to disrupt the Communist Party, and later such organizations such as the Black Panther Party, Martin Luther King, Jr.'s SCLC, the Ku Klux Klan, and others. Its methods included infiltration, burglaries, illegal wiretaps, planting forged documents and spreading false rumors about key members of target organizations. Some authors have charged that COINTELPRO methods also included inciting violence and arranging murders. In 1975, the activities of COINTELPRO were investigated by the Senate Church Committee and declared illegal and contrary to the Constitution.
Hoover amassed significant power by collecting files containing large amounts of compromising and potentially embarrassing information on many powerful people, especially politicians. According to Laurence Silberman, appointed Deputy Attorney General in early 1974, Director Clarence M. Kelley thought such files either did not exist or had been destroyed. After The Washington Post broke a story in January 1975, Kelley searched and found them in his outer office. The House Judiciary Committee then demanded that Silberman testify about them. An extensive investigation of Hoover's files by David Garrow showed that Hoover and next-in-command William Sullivan, as well as the FBI itself as an agency, were responsible.
In 1956, several years before he targeted King, Hoover had a public showdown with T.R.M. Howard, a civil rights leader from Mound Bayou, Mississippi. During a national speaking tour, Howard had criticized the FBI's failure to thoroughly investigate the racially-motivated murders of George W. Lee, Lamar Smith, and Emmett Till. Hoover not only wrote an open letter to the press singling out these statements as "irresponsible" but secretly enlisted the help of NAACP attorney Thurgood Marshall in a campaign to discredit Howard.
In the 1950s, evidence of Hoover's unwillingness to focus FBI resources on the Mafia became grist for the media and his many detractors, after famed muckraker Jack Anderson exposed the immense scope of the Mafia's organized crime network, a threat Hoover had long downplayed. Hoover's retaliation and continual harassment of Anderson lasted into the 1970s. Hoover has also been accused of trying to undermine the reputations of members of the civil rights movement. His alleged treatment of actress Jean Seberg and Martin Luther King, Jr. are two such examples.
Hoover personally directed the FBI investigation into the assassination of President John F. Kennedy. The House Select Committee on Assassinations issued a report in 1979 critical of the performance by the FBI, the Warren Commission as well as other agencies. The report also criticized what it characterized as the FBI's reluctance to thoroughly investigate the possibility of a conspiracy to assassinate the president.