The FBI has increased its reward to $50,000 for information leading to the arrest of four domestic terrorists accused of attacks in five Western states -- including a 1998 firebombing at a Colorado ski resort that caused $12 million in damage.

The four are believed to be members of a radical environmental group known as "The Family" that is blamed for a series of arsons, vandalism and other crimes in Oregon, Washington, California, Wyoming and Colorado from 1996 to 2001.

They are Josephine Sunshine Overaker and Rebecca Rubin, both Canadian citizens, and Joseph Mahmoud Dibee and Justin Franchi Solondz, both American. All are believed to be living outside the United States.

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"From 1996 to 2001, these individuals participated in a cell of approximately 20 individuals that were known as 'The Family.' They conducted a range of criminal actions through the western United States in the name of the Animal Liberation Front," Michael Ward, the FBI's assistant director for counterterrorism, said at a press conference in Washington.

The 1998 firebombing at a Vail, Colo., ski resort is considered one of the most devastating eco-terrorism attacks in U.S. history. The group also is suspected in a 2001 eco-terror arson at the University of Washington's Center for Urban Horticulture in Seattle.

The incidents were among at least 25 criminal actions by members of the Earth Liberation Front and the Animal Liberation Front that caused more than $80 million in damages, officials said.

"These individuals are terrorists. Regardless of their political or social message, their actions were criminal and violated federal laws," said Michael Ward, deputy assistant director of the FBI's counterterrorism division.

A total of 17 people have been indicted on eco-terrorism charges related to actions by the Earth Liberation Front and the Animal Liberation Front, Ward said. Thirteen have been convicted.

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FOX News' Catherine Herridge and The Associated Press contributed to this report.