"We have investigations of people in various stages of radicalizing in all 50 states," Comey was quoted as saying by The Hill website at a meeting of the National Association of Attorneys General.
His statement came after the FBI detained three men in New York City on Wednesday, who were charged with attempting to provide support for ISIL.
Comey noted that Islamic State used social media as a tool to spread its propaganda, calling on people to fight for IS "caliphate" that the group established on the territories it has seized in Syria and Iraq.
The FBI chief said that messages employed by the extremists resonate with "troubled souls" that exist in every state.
Under the circumstances, Comey stressed the importance of sharing information among local law enforcement and government agencies.
Islamic State operates primarily in Syria and Iraq. However, the extremist group is also known to be recruiting members in the United States and the European Union.
