Query: What groups did Hoover deny the existence of?
Context: Gradually the agency dismantled many of the groups. Although Hoover initially denied the existence of a National Crime Syndicate in the United States, the Bureau later conducted operations against known organized crime syndicates and families, including those headed by Sam Giancana and John Gotti. The RICO Act is still used today for all organized crime and any individuals who might fall under the Act.