Several librarians received "national security letters" from investigators and were unable to speak out during Congress' debate over the reauthorization of the 2001 anti-terrorism law, which includes the secret subpoena power. Weeks after the USA PATRIOT Act was renewed, the FBI lifted the gag order.

Leahy wondered whether Mueller thought the case involved an abuse of the NSL provision. He said a situation in which someone experiences a "real abuse" of the system and cannot talk about it is "Kafka at the extreme." Mueller said he did not believe the Connecticut case was a misuse of the system.

The Vermont Democrat also asked how many times the FBI had issued NSLs to libraries or educational institutions to date. Mueller did not have the statistics on hand and said he could provide the information by the end of the week.

Read more about the FBI oversight hearing in Technology Daily's PM edition.