But an FBI spokesman now says it appears that only one person was responsible for knocking down the towers.
And a neighbor who chased that man away from the scene with a shotgun through the predawn darkness says he saw the suspect escape down a riverbank of the nearby Snohomish River.
As the new information came in about the circumstances surrounding the tower toppling incident, the ELF continued to take responsibility for the destruction.
"Due to the health and environmental risks associated with radio waves emitted from the towers, we applaud this act by the ELF," Jason Crawford, a spokesman for the North American ELF Press Office, said in a prepared statement.
The FBI has taken the lead in the investigation, and the Snohomish County Sheriff's Office is assisting, officials said.
A neighbor who did not want to be identified told KOMO News that he was awakened at 3 a.m. by the noise of machinery a couple of hundred yards away from his house.
He said he got up and left his house in time to see one of the two towers fall. He then grabbed a shotgun and charged across the field, chasing a man he says he saw getting out of the excavator used to knock down the towers.
The neighbor told KOMO News that he was gaining on the man when he suddenly ducked into some bushes alongside the Snohomish River.
At that point, the neighbor says he decided to give up the chase rather than take the chance of also going into the bushes.
David Gomez, the assistant FBI agent in charge of the Seattle bureau, said agents searched the riverbank Friday looking for footprints or any other evidence.
Gomez said investigators have reports of only one man in the field with the towers, but it is possible that someone was waiting in a boat to help him escape down the Snohomish River.
The towers owned by family-owned radio station KRKO have been a source of controversy in the area for several years now, as opponents waged a long legal battle against the station as it sought permits for the structures east of Everett.
"We went through an extensive permitting process to get this project going, and there was some opposition in the valley that we had to deal with," said KRKO General Manager Andy Skotdal. "The testimony at the hearings was often very bitter and negative."
The station transferred its radio transmission to a backup system when the towers went down about 3 a.m. and remains on the air.
A large excavator was left tangled in the base of one of the destroyed towers, which was 350 feet tall, and the sign from ELF was on a fence nearby.
Since its formation in 1996, the ELF has become well-known for highly publicized destructive actions against corporate efforts that it views as harmful to the environment. ELF, by its own estimate, has inflicted some $150 million in damage.
Skotdal says he's not really sure if the ELF organization is to blame.
"Anybody can claim responsibility for anything," Skotdal said, adding that he doesn't really care who toppled the towers, calling it "vandalism, plain and simple."
Crawford confirmed Friday morning that the destruction was the work of the ELF.
He said there is evidence that AM radio waves cause adverse health affects including a higher rate of cancer, harm to wildlife, and that the signals have been interfering with home phone and intercom lines.
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.
Skotdal said destruction of the two towers would not prevent the station from going ahead with its plan to build the additional towers, and he expects to rebuild the two destroyed towers.
Crawford said ELF members felt this was unacceptable.
"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," Crawford said.
Skotdal said his company was working to offer a $25,000 reward for information leading to the arrest of those responsible.
"We're going to work very hard with the sheriff's department to help find these people," Skotdal said.
Some neighbors said they were not sorry to see the towers down.
Jerry and Doris Anderson, who were among those who protested the antennas, said they both woke up at about 3 a.m. by unusual noises. They told KOMO News that they heard about 12 minutes of pounding before the tower fell.
Doris Anderson said she did not advocate destruction of private property as a way of fighting the antennas.
"Obviously we fight this in the courts, not this way," she said.
But Jerry Anderson added, "I was glad this happened. It made me think of the Boston Tea Party."
Both admitted they expect to see the towers re-erected.
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.