The message on the sheet was signed with the letters “E.L.F.,” the infamous initials of the Earth Liberation Front, a loosely organized group that has been linked to multiple bold acts of ecoterrorism across the Northwest and elsewhere for two decades. Banners have claimed E.L.F. responsibility for arsons at other housing developments in the region in recent years, and the fires on Monday came as jurors deliberated in a case involving an arson in 2001 at the University of Washington that was linked to E.L.F.

Image Grey Lundberg of CMI Homes was on the scene on Monday as fires ripped through the houses he built. Credit... Stuart Isett for The New York Times

Fred Gutt, a special agent with the F.B.I.’s Joint Terrorism Task Force in Seattle, said of Monday’s fires, “There’s a claim of E.L.F. responsibility and, given that, it’s being investigated as a domestic-terrorism act.”

“But the authenticity of the claims still needs to be borne out,” Mr. Gutt added, noting the elusiveness of the Earth Liberation Front. “There’s no membership rolls. There’s no clubhouse. It’s more of an ideology. They’re organized only to the extent of maybe cells that get together and decide to act on their belief.”

None of the houses were occupied at the time of the fires, and no one was injured.

Neighbors first reported hearing what sounded like gunfire or explosions early Monday, and firefighting authorities on the scene told reporters that the houses appeared to have had multiple fires set.

The fires stunned the builders and real estate agents promoting the development, who had cast it as reflecting the “best practices” of environmentally friendly high-end home construction. They emphasized features like landscaping that requires little water, sidewalks designed to minimize runoff and reused lumber for construction.