WASHINGTON — Environmental and animal-rights activists who have turned to arson and explosives are the nation's top domestic-terrorism threat, an FBI official told a Senate committee yesterday.

Groups such as the Animal Liberation Front (ALF), the Earth Liberation Front (ELF) and Stop Huntingdon Animal Cruelty (SHAC) are "way out in front" in terms of damage and number of crimes, said John Lewis, the FBI's deputy assistant director for counterterrorism.

"There is nothing else going on in this country over the last several years that is racking up the high number of violent crimes and terrorist actions," Lewis said.

ALF says on its Web site that its small, autonomous groups of people take "direct action" against animal abuse by rescuing animals and causing financial loss to animal exploiters, usually through damage and destruction of property. ELF is an underground movement with no public leadership, membership or spokesman.

The British-based SHAC describes itself as a worldwide campaign since 1999 to rescue animals tortured in research labs and shut down the businesses that rely on their use. It says it "does not encourage or incite illegal activity."

Lewis said the FBI reached its conclusions after analyzing all types of cases and comparing the groups with "right-wing extremists, KKK, anti-abortion groups and the like."

He said most animal-rights and eco-extremists so far have refrained from violence targeting human life, but "the FBI has observed troubling signs that this is changing. We have seen an escalation in violent rhetoric and tactics," he told the Senate Environment and Public Works Committee. "Attacks are also growing in frequency and size."

Sen. James Inhofe, R-Okla., the panel's chairman, said he hoped to examine more closely how the groups might be getting assistance in fund-raising and communications from "mainstream activists" in tax-exempt organizations.

"Just like al-Qaida or any other terrorist organization, ELF and ALF cannot accomplish their goals without money, membership and the media," Inhofe said.

The FBI said it has 150 open investigations, with activists claiming responsibility for 1,200 crimes between 1990 and mid-2004.

Investigators cite examples of people using arson, bombings, theft, animal releases, vandalism, harassing phone calls, office takeovers and letters rigged with razor blades. Such tactics have been used in what officials call "direct action" campaigns to disrupt university research labs, restaurants, fur farms and logging operations. Newer targets include sport-utility-vehicle dealerships and new-home developments as signs of urban sprawl.

Officials say the incidents have caused more than $110 million in damage. The biggest so far was an arson at a five-story condominium under construction in San Diego in August 2003 that caused $50 million in damage.

In the past few years arsons and explosives have been used increasingly, Lewis said.