Craig Rosebraugh is the liaison for a more vicious brand of violence, the terrorism of the Earth Liberation Front.

Rosebraugh, a vegan baker from Portland, Oregon, fields information about arson attacks across the US done in the name of saving the planet. Although he says he has no idea who the eco-terrorists are and how they operate, he leaves his fax line open to receive anonymous communiques. The growing list of ELF vandalism, now estimated to have caused in excess of $30m in damage, is then logged on the Earth Liberation Front website, which also instructs activists to 'ignite the revolution' with a self-help guide to timer-set devices to start fires.

The ELF, styled on the Animal Liberation Front and claiming to be its sister organisation, is a breed apart even from the vandalism which has accompanied some protests. Even within radical green ranks who sympathise with the odd bit of "monkey-wrenching", the ad hoc attacks on bulldozers and construction sites in places of natural beauty, it is considered to be a step too far. (And, to be clear, the "monkeywrenchers" are a tiny minority of the hard-core environmentalists who subscribe the thinking of groups like Earth First!)

Eco-terrorists are generating a serious concern - and substantial spending - in official circles. Before September 11, the FBI had said that the ELF, which has attacked new homes being built in Long Island, logging depots in the Pacific Northwest and genetic research operations in Seattle, is the most dangerous domestic terrorist groups operating in the US today. A death as a result of an ELF attack is only a matter of time, Special Agent Steve Berry said.

Peter Chalk, a specialist in terrorism at the Rand Corporation, the think-tank which is an outgrowth of the Pentagon, was sceptical prior to the attacks on America about both about the FBI scare-mongering statements and the ELF's self-aggrandizing website. The anti-abortionist campaigners, such as God's Liberation Army, are more violent, having murdered individuals in an attempt to frighten off other doctors. In his Rand Corporation office, where he gave an interview in August, he had a picture of Osama bin Laden. This, even then, was America's most wanted man, he said.

"The one thing the law enforcement community, the government community and the intelligence community in this country does not want to hear is that terrorism is not a threat," said Chalk, noting that $10bn in federal funds are set aside each year for anti-terrorism work, of which at least $1.2bn is deployed to develop domestic defences against terrorism.

Eco-terrorism, however, fitted into the establishment's view of its vulnerabilities. In particular, eco-terrorists were seen as the type of extremists who might employ biological or chemical weapons.

According to Pentagon folklore, President Clinton read Cobra Event, a novel by Richard Preston about a group which terrorises the US by using biological weapons. Coming on the heels of the Timothy McVeigh bombing in Oklahoma and the Sarin gas attacks on the Tokyo underground, the White House worried this could be more than just fiction.

The president ordered funds to be set aside to increase the preparedness of US cities of biological, chemcial or radiological attack. The Homeland Defense Initiative, as it came to be known, poured fresh funds into a host of government agencies: FBI, Defense, Health and Human Services and the Department of Justice.

Chalk acknowledged that there were looming dangers within the violent environmental movement. The ELF is the sister organisation of the Animal Liberation Front, which has directly attacked individuals involved in animal testing.

"The environmental movement, the extremists are dangerous only in so far as they are a very unstructured movement. Some loner can tie his actions to the ideology and justify anything. There are no organizational constraints," says Chalk, suggesting another Timothy McVeigh could lurk within their ranks. "There is a real concern about biological terrorism and the environmental terrorists are seen as a real threat [on that front], because, to put it bluntly, they hate humans."

Eco-terrorism does not have broad backing, as the vast constitutency of environmentalists are non-violent. (Even the few who can stomach destruction of property shudder at the thought of any attack on life, human or animal.) It does not have organization or access to funds. It could provide the banner under which a solitary bomb thrower could mount a crazed crusade. Until September 11, the ELF was just a fringe freakshow. It was the extreme embodiment of the role of violence within the new protest movement: Contentious, overblown but unignorable.

Now, the world and, in particular, America is more frightened. The ELF and their kind are no longer just violent cranks. They will be branded as terrorists.

Even within the movement, the battle lines have been drawn. Either you are with the peaceful protesters or against them.

Part Six: Clamour against capitalism stilled

Contact James Harding at james.harding@ft.com