Eco-terrorism has become a serious US domestic security threat, a senior FBI official has told a Senate committee.
Mr Lewis said activists had adopted new violent tactics
Counter-terrorism expert John Lewis said groups like the Earth Liberation Front and Animal Liberation Front were now a law enforcement priority.
The threat posed by environmental and animal rights activists was serious and widely underestimated, he said.
He said eco-warriors committed 1,200 criminal acts between 1990 and 2004 causing millions of dollars of damage.
'New violent tactics'
"Extremists have used arson, bombings, theft, animal releases, vandalism and office takeovers to achieve their goals," said Mr Lewis, the FBI's counter-terrorism deputy assistant director.
"Investigating and preventing animal rights extremism and eco-terrorism is one of the FBI's highest domestic priorities," he told the Senate committee on environment and public works.
He said groups such as the Animal Liberation Front (ALF), the Earth Liberation Front (ELF) and Stop Huntingdon Animal Cruelty (SHAC) were "way out in front" in terms of damage and number of crimes.
"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," Mr Lewis said.
"We have seen an escalation in violent rhetoric and tactics. Attacks are also growing in frequency and size."
Mr Lewis said the FBI was committed "to bring to justice those who commit crime in the name of animal or environmental rights".
Committee chairman James Inhofe said he hoped to examine more closely the groups' financing from mainstream sources not implicated in the violence.
"Just like al-Qaeda or any other terrorist organisation, ELF and ALF cannot accomplish their goals without money, membership and the media," Mr Inhofe said.