Two radio station towers north of Seattle were toppled early Friday, a banner left nearby bore the initials of the Earth Liberation Front and an FBI spokesman said authorities haven't found any evidence that other individuals or groups besides the ELF were involved.

The towers were torn down because of health and environmental concerns, according to an e-mail from the North American ELF Press Office, which has represented the shadowy group in the past.

"We have to weigh our priorities, and the local ecosystem in Everett, along with the local residents, do not need additional sports news radio station towers that come at the expense of reduced property values and harmful radio waves," ELF press office spokesman Jason Crawford said in the e-mail.

The ELF is a loose collection of radical environmentalists that has claimed responsibility for dozens of attacks since the 1990s. The FBI has taken over as lead investigator in Friday's incident.

The towers apparently were taken down with a track hoe, a piece of heavy construction equipment that was already on the site, FBI agent Marty Prewett said.

The ELF's Web site featured a picture of one of the towers lying on its site with the caption "Earth Liberation Front Topples Two Radio Station Towers in Snohomish County, WA," followed by the words: "Details coming soon."

Crawford told the AP by telephone he believed more than one person was involved. He said he had not spoken directly with anyone who claimed to be involved but received some e-mail from what appear to be locals who support the action.

The towers belonging to KRKO — a family-owned station in Everett, about 25 miles north of Seattle — have prompted complaints from neighbors of interference from radio signals on home telephone and intercom lines. The site for the towers is in the town of Snohomish, about eight miles southeast of Everett.

The station's plans to increase its transmission capacity by building more towers on the site have been embroiled for more than a decade in appeals and litigation over issues ranging from trumpeter swan habitat to potential health hazards to humans.

A neighbor told a 911 operator that someone seemed to be attacking the towers with a bulldozer or other heavy equipment at about 3:30 a.m., Snohomish County sheriff's office spokeswoman Rebecca Hover told The Herald of Everett.

Sheriff's deputies on Friday morning found a 349-foot tower and a smaller tower on the ground with heavy construction equipment nearby, Hover said.

Investigators have found no evidence of injury, Prewett said.

The station remained on the air Friday after shifting to other transmission equipment. A $25,000 reward for information leading to the capture of whoever was responsible was announced by Andy Skotdal, the station's general manager.

"We'll use our airwaves to do it, too," Skotdal told The Herald.

The ELF has claimed responsibility for several arsons in the region, including a fire that destroyed the Center for Urban Horticulture at the University of Washington in Seattle in 2001.

An ELF sign was left at the scene of fires set on March 3, 2008, at a number of luxury houses in a development in Echo Lake, north of Seattle.

KRKO's additions to its existing towers in Snohomish were completed in February, allowing the station to boost its AM signal to where it could compete with larger broadcasters in the Seattle-Tacoma area. That's when the complaints from neighbors over telephone interference began.

The Skotdal family also plans to build two 199-foot towers at the same site for a new 5,000-watt AM station that would cover Snohomish County on another frequency.

A hearing examiner denied a permit for the towers, based on claims that radio signals could be dangerous to humans. But the council voted to reverse the finding, saying it was based on shaky scientific evidence.

A King County judge upheld the council's decision on Aug. 14.

"When all legal channels of opposition have been exhausted, concerned citizens have to take action into their own hands to protect life and the planet," Crawford said in the e-mail.

Skotdal has said he hopes to get the new signal on the air by the end of the year. The towers being built apparently escaped damage.

The action upset one of the leading opponents of KRKO's expansion plans.

"It has been a long legal battle and I'm upset to see this kind of violence happen here," Lee Bennett Jr., president of Citizens to Preserve the Upper Snohomish River Valley, told The Herald. "This is not the way to handle it."

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