A small explosive device was set off over the weekend, causing up to $30,000 in damage to each home. No one was injured.
"This hopefully provided a firm message that we will not tolerate the destruction of our island," the Earth Liberation Front said a statement faxed to The Associated Press on Sunday.
Police also believe the ELF is responsible for millions of dollars in arson and vandalism to six other unoccupied homes on Long Island, a region that stretches east of New York City, over the past month.
Police, the district attorney's office and the Federal Bureau of Investigation have formed a task force to investigate the fires and identify the members of the loosely constructed environmental group, headquartered in Portland, Ore.
"This is not an environmental action. This is a criminal act," Suffolk County Police Commissioner John Gallagher said Wednesday. "I am concerned with their latest statement that everything we build that they deem environmentally sensitive, they will destroy."
The Long Island Builders Institute has offered a $10,000 reward for the arrest and conviction of those responsible for the weekend vandalism.
The ELF, acting at times with the Animal Liberation Front, has claimed responsibility for dozens of actions across the country since 1996, including a 1998 blaze at a Vail, Colo., ski resort that caused $12 million in damage. The environmental group said the expansion project threatened lynx.
"They want to stop endless devastation of the American landscape by over-development," said Craig Rosebraugh of Portland, Ore., who identifies himself as ELF's spokesman but says he is not a member. He said the group's leaders communicate with him by a variety of means that preserve their anonymity.
The homes targeted over the weekend at the Island Estates Development were being built on a former peach farm. A message had been scrawled in red paint on one of the houses: "If you build it, we will burn it."
In 1998, Suffolk County had 34,000 acres of agriculture land. At the current rate of development, land-use experts say, only 9,000 acres will remain by 2015.
Another environmental group dedicated to saving Long Island's open space called the damage "worse than pointless."
"They don't have to commit random acts of violence to let people know that Long Island's overdeveloped, since everyone knows we are," said Dick Amper, executive director of the Long Island Pine Barrens Society.
"Should we burn down people's houses about it? No. We should probably throw some politicians out of office," he added.
The ELF, which maintains a Web site out of Canada, also claims to have caused millions in damage nationide.
"We are trying to cause the rich sprawl corporation enough money so they stop destroying the planet, and thus the health and well-being and existence of humankind," the group's statement said. "The Earth isn't dying, it's being killed. And those who are killing it have names and addresses."