WASHINGTON (AP) - A radical environmental group that has carried 600 attacks since 1996 has become the largest and most active U.S.-based terrorist group, the FBI's top domestic terrorism officer said Tuesday.

But a House committee's efforts to shed light on the Earth Liberation Front and its companion, the Animal Liberation Front, were frustrated when former ELF spokesman Craig Rosebraugh refused to answer questions from members of Congress.

``I'll take the Fifth Amendment,'' Rosebraugh said more than 50 times to questions ranging from whether he helped produce an ELF training film to who was paying for his attorney.

Rosebraugh was subpoenaed to testify at the request of Rep. Scott McInnis, R-Colo., chairman of the House Subcommittee on Forests and Forest Health.

Rosebraugh has said he relayed anonymous messages on ELF's behalf from 1997 until he quit last September, but had no firsthand knowledge of any attacks.

FBI expert James F. Jarboe said that since 1996, the ALF and ELF have caused $43 million in damage in more than 600 attacks, ranging from spray-painting buildings and breaking windows to firebombing fur farms, research centers and a ski resort.

``They're the most active. They cause the most damage,'' Jarboe said, although white supremacist groups are still considered more dangerous because their attacks are often aimed at people.

Nobody has been killed in an ELF or ALF attack, but McInnis said it is wrong to think of the ecoterrorists as ``nature-loving hippies'' or misguided youths.

``These are hardened criminals,'' he said. ``They are dangerous, they are well-funded, they are savvy, sophisticated and stealthy, and if their violence continues to escalate, it is only a matter of time before their parade of terror results in a lost human life.''

In a report the ELF and ALF issued last month, the groups claimed responsibility for 67 illegal actions in 2001, including burning down a $5.4 million horticulture building at the University of Washington.

In 1998, the ELF claimed responsibility for an arson attack at Vail Ski Resort that did $12 million in damage.

Rep. Jay Inslee, D-Wash., said he understands that die-hard environmentalists are frustrated with some of the Bush administration's policies, but ecoterrorism hurts the cause by making it harder for him and others to promote more environmental-friendly policies.

Asked about Inslee's criticism after the hearing, Rosebraugh said, ``I was forced to come to Washington, D.C. I'm not going to answer that question.''

Rosebraugh's attorney, Stuart Sugarman, said he wanted ``to thank Mr. McInnis for providing attention for this important cause.''

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