John Lewis, the agency's deputy assistant director for counter-terrorism, told a senate committee in Washington that the militant groups were "way out in front" in economic damage. He also suggested it would not be long before loss of human life was added to their tally of crimes.

"There is nothing else going on in this country over the past several years that is racking up the high number of violent crimes and terrorist actions," he told senators, prompting some Democrats on the panel to warn against tagging all environmental and animal rights organisations with the same label.

The hearing, held by the Senate Environment and Public Works Committee, was an unusual detour from the US government's more familiar preoccupation with terrorist threats from foreign groups such as al-Qa'ida. The FBI said it has 150 investigations into 1,200 crimes allegedly committed by eco-terrorists and animal welfare militants between 1990 and 2004.

Of most interest to the authorities are two US-based groups, the Animal Liberation Front (ALF) and the Earth Liberation Front, (ELF). But Mr Lewis also highlighted the British-based Shac, or Stop Huntingdon Animal Cruelty. Founded to cripple animal testing at the Huntingdon Life Sciences laboratory, it describes itself on its website as a worldwide organisation also active in the US.

Shac says it "does not encourage or incite illegal activity". On its website, ALF says members take "direct action" to prevent animal cruelty. It has allied itself with the more shadowy ELF, which has no website and stays underground but has sometimes claimed responsibility for actions.

Tactics attributed to the groups include arson attacks, letters put in the post with razor-blades, bombings and office takeovers. They were blamed most notably for an arson attack on an apartment complex in San Diego in 2003 that caused $50m (£27m) in damage.

Among the groups' aims seems to be the discouragement of urban sprawl. This week, police were blaming eco-terrorists for blazes at three places on Long Island, including a suspicious fire that swept through a new housing development in the exclusive Oyster Bay community.

The chairman of the senate committee, the Republican James Inhofe of Oklahoma, added to the sense of alarm, agreeing with Mr Lewis that killings might be next. "The danger of ELF and ALF is imminent," he said. "Although they have not killed anyone ... it is only a matter of time until someone dies as a result of ELF and ALF criminal activity."

And evoking language more normally heard in connection with al-Qa'ida, Senator Inhofe urged the FBI to seek out the money pipeline funding the groups.

But there were objections from Democrats to some of the testimony and particularly to Senator Inhofe managing to mention the militant groups such as ELF and the thoroughly mainstream Peta, People for the Ethical Treatment of Animals, in the same breath. (Peta may throw the occasional pot of paint at fur-sporting models, but has hardly been labelled a threat to national security.)

"I deplore as much as anybody here these violent acts," Senator Frank Lautenberg of New Jersey said. "But I am against this loose characterisation that takes innocent people and throws them in with a bunch of thugs."

Separately yesterday, the American Civil Liberties Union announced that it was suing the FBI for allegedly "over-spying" on political activists, even if their aims were peaceful, and refusing to release information about investigations, all in the name of homeland security.

The boy who firebombed Hummers

Billy Cottrell: student, 23, jailed for eight years

William, or "Billy", Cottrell, a physics student who was sentenced to eight years in prison for firebombing a Hummer dealership and damaging 125 SUVs in August 2003, is exactly the kind of eco-militant the FBI is worried about.

The 23-year-old graduate student was found guilty last month of helping to destroy car dealerships, in a series of co-ordinated attacks around Los Angeles, spray-painting the "gas-guzzling" vehicles with slogans such as "Fat, Lazy Americans" and "I * pollution".

Cottrell also admitted to tagging the vehicles with the letters ELF, the initials of radical group, the Earth Liberation Front, which has claimed responsibility for a string of arsons in Detroit, Philadelphia and San Diego.

The ELF website lauds Cottrell as an "environmental campaigner"; he denies he is a member of the group and, while standing trial, told Judge R Gary Klausner he wanted "nothing more than to be a physicist" and was never "even going to jaywalk again".

His defence lawyer, Michael Maycock, said Cottrell, a student at the University of Chicago and a doctoral candidate at the California Institute of Technology in Pasadena, was highly intelligent and had read Einstein's theory of relativity in his early teens.

"I can't understand why he changed from Einstein to Frankenstein," said Ziad Alhassen, owner of the Hummer dealership. "I don't understand why he changed from velocity to violence."

Defence and prosecution experts agreed Cottrell suffered from Asperger's syndrome, a form of autism that affects a person's capacity for social interaction.

His defence team claimed the condition led Cottrell to be duped into the SUV attacks by his two associates, who are both considered to be fugitive activists.

But the court eventually agreed with the prosecution that Cottrell was an eco-terrorist who showed no remorse for the damage he had done.

His two-and-a-half hour crime spree was described as "horrendous" by Judge Klausner, who ordered him to pay $3.5m (£2m) in damages.

Cottrell's lawyers have launched an appeal against the conviction.

Elizabeth Davies