After six years of active duty, including a stint as a military police officer, Hatton joined the FBI in New Orleans in 1985, and worked briefly in a small Louisiana office. In 1991, he was assigned to New York City as a member of an investigative team working on a bank robbery case. Leonard Hatton's main job was an FBI specialist in explosives and evidence recovery, where he worked until his death. In addition to cracking a number of high-profile bank robbery and kidnapping cases, he investigated international terrorist attacks such as the 1993 World Trade Center bombing, the 1998 United States embassy bombings, and the 2000 USS Cole bombing. Three months before the September 11 attacks, he had testified in the trial of a follower of the Islamic terrorist group al-Qaeda.