"Terrorism does remain the FBI's top priority, but in the not too-distant-future we anticipate that the cyberthreat will pose the greatest threat to our country," Mueller told attendees at the annual cryptography and information security-related RSA Conference.
"Today, terrorists have not used the Internet to launch a full-scale cyberattack, but we cannot underestimate their intent," CNN reported him as saying.
In its increasing preparations to fight cybercriminals, the FBI has installed a dedicated cybersecurity squad in each of its 56 field offices and has 1,000 agents and analysts keeping an eye on the Web, he said.
The FBI is now focusing on three primary threat sources, he said: terrorists, organized crime rings and state-sponsored cyberespionage.
Mueller, while not mentioning China by name, acknowledged that country's prominent presence on the threat landscape.
"State-sponsored hackers are patient and calculating," Mueller said. "They have the time, money and resources to burrow in and wait. You may discover one breach only to find that the real damage has been done at a much higher level."