Explosive devices were found inside luxury houses set ablaze Monday morning outside of Seattle, and police suspected that a well-known eco-terrorism group ignited the fires.
Meanwhile, a developers association offered a $100,000 reward for information leading to those who committed the apparent arson.
The multi-million-dollar development known as "Street of Dreams" in Woodinville, Wash., burst into flames in the early morning hours, and Snohomish County crews fought to contain the blaze throughout the day Monday.
The Earth Liberation Front, known for violent acts in the name of environmentalism, left a sign at the scene and was suspected to have set fire to the swanky, newly built neighborhood — though authorities hadn't ruled other possible culprits out.
The Federal Bureau of Investigation released a statement saying that the FBI, the Bureau of Alcohol, Tobacco and Firearms (ATF) and local authorities were on-scene of the possible arson.
"This is being investigated as a domestic terrorism act by the Seattle FBI's Joint Terrorism Task Force, but it is too early in the investigation to make any determination," the FBI said Monday in a statement.
An ex-official with the ATF, who isn't involved in this investigation, told FOXNews.com that it was important not to jump the gun on who was behind the apparent crime — especially with an unstructured and disjointed group like the ELF.
"In many cases, someone will claim responsibility or leave literature behind, but law enforcement doesn't necessarily exclude all others," said ATF former assistant director Michael Bouchard. "They let the evidence take them to the guilty parties."
But Bouchard said it was typical, as in this case, for the ELF to leave a calling card behind.
The Seattle-area houses burned as a federal jury in Tacoma was about to resume deliberations in the case of an alleged ELF activist, Briana Waters. Waters could face at least 35 years if convicted of helping to firebomb the University of Washington's Center for Urban Horticulture in 2001.
No injuries were reported in the Monday fires, which began before dawn in the wooded subdivision and were still smoldering by midmorning.
The Building Industry Association of Washington said it was offering a $100,000 reward for the arrest and conviction of the Earth Liberation Front terrorists who apparently started the fire.
"While ELF used to spike trees and sabotage logging equipment, they're now burning down homes," said BIAW president Brad Spears, adding it was only a matter of time before someone is hurt or killed by ELF terrorists. The BIAW represents 13,500 companies involved in the homebuilding business.
The Snohomish County District Seven Fire Department, which was extinguishing the flames, said Monday that the case had been turned over to the Snohomish County Sheriff's Office — which estimated damage at $7 million. In addition to the three uninhabited show homes destroyed, two sustained smoke damage.
Explosive devices were found in the homes, and crews were able to remove them, said Fire Chief Rick Eastman of Snohomish County District 7.
The houses are in a development near the headwaters of Bear Creek, which is home to endangered chinook salmon. Opponents of the development had questioned whether the luxury homes could pollute the creek and an aquifer that is a drinking water source, and whether enough was done to protect nearby wetlands.
The sign found at the scene bore the initials "ELF" — those of the radical environmental group — and mocked claims that the luxury homes were environmentally friendly, according to video images of the sign aired by KING-TV.
"Built Green? Nope black!" the sign said.
The sign, a sheet with red scraggly letters, said "McMansions in RCDs r not green," a reference to rural cluster developments.
One of the people involved in the project said the homes used "green" techniques such as water-pervious sidewalks, super-insulated walls and windows and products made with recycled materials, such carpet pads. Advertising for last summer's Street of Dreams show focused on the environmentally friendly aspects of the homes, which were smaller than some of the huge houses featured in years past.
"It's very disappointing to take a situation where we're tying to promote good building practices — Built Green practices — and that it's destroyed," said Doug Barnes, the Northwest division president of Centex Homes in Kirkland.
The fires started at a strip of unoccupied, furnished luxury model homes where developers show off the latest in high-end housing, interior design and landscaping. The homes are later sold, though the ones that were set on fire Monday hadn't been yet. The homes that burned were between 4,200 and 4,750 square feet, with prices up to nearly $2 million.
The blazes are suspicious because they were set in multiple places in separate houses, said Eastman — who confirmed that the ELF sign was found at the scene of the fires in the community north of Woodinville, where some homes were still under construction.
The Earth Liberation Front is a loose collection of radical environmentalists known for trying to cause economic damage to companies or organizations that, in its opinion, harm the environment. The group has no organized structure or leadership; typically, autonomous cells of activists take "direct actions" such as arsons and claim responsibility on behalf of ELF.
The group has been a menace in the Northwestern United States, according to experts.
"It's a pretty serious problem," Bouchard, the former ATF assistant director, told FOXNews.com. "Fortunately for law enforcement, they've been pretty successful in arresting them." Many of those arrests, he said, have turned into convictions.
Waters, a 32-year-old violin teacher from Oakland, Calif., is accused of serving as a lookout while her friends planted a devastating firebomb at the UW's horticulture center in 2001, causing $7 million in damages. The horticulture center was targeted because the ELF activists mistakenly believed researchers there were genetically engineering trees, investigators said. No one was hurt in the arson.
That fire was one in a string of arsons that investigators say were perpetrated from the mid-1990s to 2001 by ELF.
In 2005, federal authorities charged more than a dozen people involved in an ELF cell known as "the Family" that was centered near Olympia, Wash., and Eugene, Ore. The group was responsible for at least 17 fires around the West from 1996 to 2001 — most notoriously, the 1998 destruction of the Vail Ski Resort in Colorado, a fire that caused $12 million in damage.
FOX News' Catherine Donaldson-Evans, Ian McCaleb and The Associated Press contributed to this report.