At a congressional hearing on counterterrorism, Freeh cited ''rogue terrorists'' such as Saudi dissident Osama bin Laden, whom Washington has accused in the bombing of two US embassies last year in East Africa, as probably the most urgent risk to US interests.
But Freeh said the domestic threat could not be ignored, especially as the millennium approaches.
''The possibility of an indigenous group like Aum Supreme Truth cannot be excluded,'' he said, referring to the cult held responsible for a nerve-gas attack in the Tokyo subway system in March 1995.
''With the coming of the next millennium, some religious apocalyptic groups or individuals may turn to violence as they seek to achieve dramatic effects to fulfill their prophecies,'' he said.
Freeh expressed dismay at a ''pattern of racist elements'' seeping into the US militia movement, most of which, he said, had no racial overtones and did not espouse bigotry.
But he discussed at length ''a disturbing trend'' toward Christian Identity and other hate philosophies that provided both a religious base for racism and anti-Semitism, as well as an ideological rationale for violence against minorities.
''Many white supremacist groups adhere to the Christian Identity belief system, which holds that the world is on the verge of a final apocalyptic struggle ... and teaches that the white race is the chosen race of God,'' he said.
Many of those who believe in this credo are engaged in survivalist and paramilitary training, storing food and supplies and hiding weapons.
As 1999 comes to a close, Freeh said, Identity's more extreme members could prepare for Armageddon by carrying out armed robberies to finance a battle, destroying government property, and targeting Jews and nonwhites.
The FBI, Freeh said, has ''little credible intelligence'' at this time indicating that terrorists, either domestic or international, were preparing to attack the United States.
But he added that ''a growing number, while still small, of `lone offender' and extremist splinter elements of right-wing groups have been identified as possessing or attempting to develop or use'' chemical, biological, or nuclear weapons.
Attorney General Janet Reno, who also appeared before the Senate Appropriations subcommittee, said a terrorist attack using a biological weapon might not be immediately apparent but could have far-reaching impact on victims and emergency personnel.
''In fact, we have found recently the mere threat of the use of unconventional weapons can cause concern and panic. Threats to release harmful biological or chemical substances cannot be ignored,'' she said. Freeh said the FBI dealt with an ''anthrax warning letter'' somewhere in the country almost every day.
Federal officials had to be able ''to match wits with the bad guys,'' Reno said.
She appealed to the Senate panel to approve funds for a National Domestic Preparedness Office to be led by the FBI to provide coordination and a single point of contact for state and local communities.