Washington ? A radical environmental group that has carried out 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.

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

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.