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It began 16 years ago with a meeting of disaffected environmentalists in Brighton. Today the radical organisation they created - the Earth Liberation Front - is described as a decentralised al-Qaeda-style network and America's No 1 domestic terrorism threat. It even provided inspiration for the villains in Michael Crichton's 2004 thriller State of Fear.

In a week in which one alleged member of the ELF was convicted of helping to destroy a $7 million (£3.5 million) University of Washington research facility, and others were accused of firebombing the Street of Dreams - a development of luxury eco-mansions in Seattle worth $7 million - concerns are rising that the US could eventually face a greater threat from radical anti-global-warming activists than from radical Islamists.

Others say that the threat is overblown and designed to tarnish mainstream environmental groups that find themselves campaigning against what they regard as America's profligate consumption of resources.

According to the FBI, the ELF - the members of which are known unofficially as the Elves - has been responsible for more than 1,200 criminal acts in the US, with the cost of their “ecotage” close to $100 million. Some estimates put the figure at twice that amount.

The ELF seemed to have gone quiet since its last spate of attacks, which included a $5 million campaign of Hummer-bombings on the West Coast in 2003; a $50 million arson attack on a newly built apartment building in San Diego in the same year; and the $12 million firebombing of the upmarket Vail ski resort in Colorado in 1998. Now they are back, just as concern over climate change and the consumption of natural resources has become almost a mass hysteria.

The initials ELF were spray-painted in red on the Street of Dreams eco-mansions destroyed by fire this week. So far, no one has been arrested in connection with the attack. “If you stop and think about the lives that you're touching when you do something like this, it's unbelievable selfishness,” Grey Lundberg, who built the homes, said. “It's so ironic. We were trying to demonstrate a better way to build.”

The houses, located near wetlands and a creek where an endangered species of salmon can be found, had been designed to showcase green materials and technologies such as recycled wood and superinsulation.

When the ELF was created in 1992 by members of Britain's law-abiding Earth First! group, it was designed to operate under the same leaderless, hard-to-prosecute, resistance-style principles as the Animal Liberation Front, which also engaged in direct action and sabotage. Within two years ELF cells had emerged in continental Europe and by the mid-1990s they had spread to Canada and the US.

“There are over six billion people on this planet, of which almost a third are starving or living in poverty,” an early ELF statement said. “Building homes for the wealthy should not even be a priority. The time has come to decide what is more important: the planet and the health of its population or the profits of those who destroy it.”

Often, ELF members turn out to be unlikely terrorists. On Thursday Briana Waters, 32, a violin teacher from Oakland, California, was convicted of serving as a lookout while her co-conspirators firebombed the University of Washington Centre for Urban Horticulture, which they mistakenly believed was responsible for genetically modifying poplar trees.

In the 2003 Hummer-bombing case, a postgraduate physics student at the California Institute of Technology was prosecuted, with his lawyers claiming that the ELF preyed on the young and impressionable to carry out their attacks. They said that the student suffered from Asperger's syndrome, a form of autism. The FBI had previously focused on another cell, known by the Charles Mansonesque title of The Family, charging more than a dozen members based in Washington and Oregon with 17 fires. The FBI says that only luck has prevented anyone being killed in ELF attacks.

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