When J. Edgar Hoover became Director of the Justice Department's Bureau of Investigation in 1924, all it did was investigate federal crimes. It had been founded by Teddy Roosevelt in 1908 to investigate corruption in government. By the time Hoover took over, the range of federal interests in crime had grown, but agents were still required by law to report suspects to local or state law enforcement officials. Bureau of Investigation agents were prohibited from making arrests or even carrying weapons without being deputized by local authorities. 

During the 1920s, federal policing was not highly regarded. Americans were afraid a national police force could become a secret police. Federal enforcement of Prohibition was inconsistent, underfunded by Congress -- and resented more and more by citizens who objected to government intrusion. Finally, the Bureau's involvement in the Teapot Dome scandal, in which oil-rich Wyoming public lands were exploited by private companies acting in collusion with Cabinet officials, gave all federal policing a bad name as the story of government corruption unfolded between 1924 and 1928. 

By 1924, Americans — including American criminals — had found new mobility. By that date, the Ford Motor Company had already produced 10 million of its popular Model T cars. More crimes occurred across state lines, and criminals used jurisdictional limitations to evade the law. Support for a national police force grew in response to these crimes and to increasingly corrupt local police systems. On November 24, 1932, the Bureau of Investigation started the first national crime laboratory in the United States. Besides serving the B.O.I., its mission was to assist local law enforcement agencies with ballistics testing and fingerprint identification. In his memoirs, Special Agent Melvin Purvis wrote that by 1936, the FBI had the fingerprints of 6 million criminals on file in Washington. In the 1930s, fears of secret policing were still rampant, particularly as Americans observed that federal policing was part of the totalitarian machinery in Germany and the Soviet Union. To allay the public's misgivings, the FBI made an elaborate public relations effort to reassure people.

On August 10, 1933, the B.O.I. was officially renamed the Division of Investigation. At a time when law enforcement was becoming more professional, Hoover continued his campaign to create a modern, professional force that combined scientific methods with advanced police skills. He insisted on stringent qualifications for D.O.I. recruits. They had to have a college or law school degree, as well as a background in law enforcement. They were trained in forensic science, as well as the use of firearms. Only white men were allowed to become special agents; all women and minority men were excluded. 

Hoover expected military discipline from his force. "Instructions," he wrote, "should be obeyed by the Bureau's agents without question and immediately." Special agents even had their desks inspected regularly, and were disciplined for coming to work late. Once Melvin Purvis complained that he was late only because the office clock was fast. An inspector wrote, "This man's attitude is not exactly right and he should receive close supervision until such time as it is determined whether or not he is breaking other rules of the department which he does not believe are fair rules."

Most special agents were not street-wise and initially made many mistakes. While attempting to capture the Dillinger gang at Little Bohemia, Wisconsin, on April 22, 1934, special agents accidentally killed an innocent man and wounded two others. In his memoirs, Purvis gave a detailed account of the incident, blaming the urgency of the situation for the lack of preparation. He wrote, "It is true that during the last several years the Federal Bureau of Investigation has been organized on an emergency basis..." Purvis also blamed understaffing. In spring 1934, the D.O.I. had only about 400 special agents.

On May 18, 1934, Roosevelt signed a bill giving the federal government more power to fight crime. At the ceremony, he said, "I stand squarely behind the efforts of the Department of Justice to bring to book every lawbreaker, big and little." The bill lifted restrictions preventing federal agents from making arrests and carrying arms. 

When special agents killed John Dillinger on July 22, 1934, they became instantly famous. Purvis, head of the Chicago office, became a national hero. Confidence in the D.O.I. soared. By making the country safe from public enemies, they represented the New Deal, and the federal government's restoration of a nation that had been on the brink of collapse.

Popularly known as G-men, special agents appeared in detective magazines, on radio shows, and in movies. The movie G-Men, which appeared in 1935, changed the balance of power in the Department of Justice. By focusing entirely on special agents and their Director, it ignored Attorney General Homer S. Cummings. Until then, Cummings had coordinated a strategy combining public awareness and legislation, as well as police power, to eradicate crime. Due to the success of G-men and other FBI films, the public gave credit to the FBI alone. The agency became so popular that tourists requested tours of its Washington headquarters, particularly the crime laboratories and firing ranges.

On July 1, 1935, the D.O.I. was renamed the Federal Bureau of Investigation. Cummings disappeared from view, as the American people credited the FBI with inventing forensic science. Although the FBI was still officially an organ of the Justice Department, it functioned independently, and became known as the world's largest, most modern national police force. J. Edgar Hoover was Director of the agency for 48 years, until his death in 1972 at the age of 77.