Hoover began using wiretapping in the 1920s during Prohibition to arrest bootleggers. In the 1927 case Olmstead v. United States, in which a bootlegger was caught through telephone tapping, the United States Supreme Court ruled that FBI wiretaps did not violate the Fourth Amendment as unlawful search and seizure, as long as the FBI did not break into a person's home to complete the tapping. After Prohibition's repeal, Congress passed the Communications Act of 1934, which outlawed non-consensual phone tapping, but allowed bugging. In the 1939 case Nardone v. United States, the court ruled that due to the 1934 law, evidence the FBI obtained by phone tapping was inadmissible in court. After the 1967 case Katz v. United States overturned the 1927 case that had allowed bugging, Congress passed the Omnibus Crime Control Act, allowing public authorities to tap telephones during investigations as long as they obtain a warrant beforehand.

What tool did Hoover use to find bootleggers?