Monday's firebombing of a residential construction site, the third occurrence of suspected eco-terrorism in the Sacramento area in less than two months, comes after the eco-terror group Earth Liberation Front (search) sent an ominous letter to area newspapers warning of such actions every few weeks.

In the latest incident, firebombs exploded at an apartment complex. Because the sprinkler system worked, the fire was extinguished. Several residents were home at the time but not hurt.

This is the third incident where the ELF left graffiti messages saying, "We will win," "Leave" and "You will pay."

In a letter to local newspapers, ELF claimed it would burn down buildings "every few weeks" until families and businesses stop moving to suburbs.

"The latest missive of the ELF in Sacramento (search) is quite alarming because it hints around at targeting people," said Kelly Stoner, executive director of Stop Eco-Violence! SEV is dedicated to "exposing the true nature of eco-terrorism and building the broad support necessary for public officials and law enforcement to effectively stop it," according to the grassroots group's Web site.

The ELF is a vigilante group. Since 1996, members have torched university research centers, government buildings, a ski resort, SUV dealerships, private homes and apartments.

The FBI (search) is investigating ELF as a terrorist organization, but the group is difficult to penetrate because members often act alone.

"They are very loosely affiliated and they are not the type of organization that comes to town hall meetings and they all get together and talk about what they want to do," FBI agent Keith Slotter said.

The ELF did not respond to FOX News e-mails, but according to its Web site, www.earthliberationfront.com, "Individuals who choose to do actions under the banner of ELF do so only driven by their personal conscience. These have been individual choices and are not endorsed, encouraged, or approved of by the management and participants of this Web site."

One challenge in stopping them is that so far the courts have treated their actions as property crimes, not terrorism.

Click on the video box above for a complete report by FOX News' William La Jeunesse.