A sign fire officials found at the scene north of Woodinville, Wash., was reportedly left by the Earth Liberation Front, a radical environmental group, Seattle television station KING reported.

The sign read: "Built Green? Nope Black! McMansions + RCD's (rural cluster developments) r not green. ELF"

No injuries were reported; the homes were empty, the TV station reported. Besides the three gutted homes, a fourth structure sustained heat damage and the fifth was untouched.

The five homes were on the "Street of Dreams," where contractors build luxury, multimillion-dollar homes that include many amenities. The money people spend to tour the homes is donated to charity.

Fire officials said they were letting the fires burn because they feared the homes were booby-trapped. Officials said they heard explosions in the homes.

In 2001, the FBI classified ELF as the top domestic terror threat in the United States and categorized its followers as "eco-terrorists," previously published reports said. The group has taken responsibility in the past for similar events.