Fire ripped through luxury homes under construction in Washington state. Molotov cocktails ignited sport-utility vehicles in Virginia. Graffiti coated a dozen SUVs in Santa Fe, N.M. All in the name of Mother Earth -- and all apparently the work of a shadowy environmental group known as the Earth Liberation Front (ELF).

In its ongoing guerrilla campaign to promote its environmentalist agenda, ELF has claimed responsibility for more than a dozen acts of sabotage across the nation in the past year, causing more than $60 million in damage. ELF sees unbridled capitalism leading to uncontrolled suburban sprawl and believes the SUV epitomizes the over-consumption that is running amok in the United States. Federal and local law enforcement authorities say the group's attacks are becoming more frequent and more destructive.

No longer content to direct their wrath at lumberjacks cutting trees deep in the forest, members of ELF -- known as elves -- are increasingly targeting suburbia, especially the new developments that are encroaching on wilderness.

When a $50 million condominium complex under construction in San Diego was torched last summer, in what authorities are calling the most destructive act of eco-terrorism on U.S. soil, investigators discovered a banner at the site. It read: "If you build it, we will burn it." It was signed: "The E.L.F.s are mad."

The loosely organized underground movement is continuing to frustrate law enforcement officials across the country. Only a few arrests have been made in the past five years, and there have been even fewer convictions. Hundreds of thousands of dollars in rewards have been offered by authorities, and additional FBI agents have been assigned to investigations involving the group.

Unlike traditional criminal enterprises such as the mob or biker gangs with rigid hierarchies that have been successfully infiltrated by undercover agents, there is no formal club for officials to break into.

Federal investigators and prosecutors have expressed frustration at the difficulties of cracking down on multi-state campaigns of low-level crimes, intimidation and property damage because of a lack of applicable federal laws.

"Law enforcement is geared to catch the common criminal. These people are criminals, but they are not of the common variety," said Gary R. Perlstein, a professor in the administration of justice division at Portland State University. "They've read the law enforcement manuals. They try not to make mistakes."

ELF's Web site offers links to wide-ranging instructions, from how to build Molotov cocktails to recognizing surveillance by government agents to encrypting computer activity. Arson is the weapon of choice.

On its Web site, ELF invites people to commit acts of destruction and e-mail the group detailing their deeds. A claim of responsibility appears on the Web site a few days later. The group urges members to take all necessary precautions against harming humans and animals.

Ron Arnold, the author of several books on the environmental movement who coined the term eco-terrorism, said that delaying a claim of responsibility for a few days after the incident is a strategic move by the group to make sure no one was hurt or killed.

The FBI has been studying the group's recruiting efforts in recent years. "These tend to be younger, college-age students with a typically ideological thought process, and they have an idea of the way things should be," said Robert Blecksmith, chief of the FBI's domestic terrorism section.

ELF is divided into cells, ranging from three to 10 members, said Craig Rosebraugh, the group's former spokesman. He said in an interview that members of one cell have no contact with members of another so that they cannot implicate others.

In August, vandals claiming to be members of ELF were caught on surveillance tape spray-painting 125 SUVs at car dealerships and homes in the Los Angeles area. The vehicles, which were also firebombed by unknown assailants, were splashed with slogans such as "Fat, Lazy Americans" and "I {heart} pollution." Two suspects have fled the country, but agents arrested California Institute of Technology physics graduate student William Jensen Cottrell.

Cottrell, 23, has denied being an ELF member.

"I'm not an environmental activist," Cottrell said in an interview at the San Bernardino Central Detention Center. "I'm just environmentally aware. I support some illegal actions for environmental causes basically because writing letters doesn't work. Who are you going to write a letter to? Arnold Schwarzenegger, who owns six Hummers, or George Bush, who comes from the oil industry?"

When he found out he was being investigated by the FBI, Cottrell contemplated fleeing but decided to stay put. He pleaded not guilty in March to nine counts of arson and conspiracy, and faces as much as 35 years in prison if convicted. The trial is scheduled for October.

"If you just run, nothing happens. This way, more attention gets drawn to the problems with the government and their environmental policy," Cottrell said. "Of course, if I get 30 years, I'll think, damn, why didn't I run?"

In one of the most recent prosecutions, a federal jury acquitted Connor Cash, 22, of arson and terrorism charges last month. He had been charged with damaging five homes under construction on Long Island, N.Y. Two admitted ELF members pleaded guilty to conspiracy charges in connection with that incident.

In March, Canadian authorities arrested Tre Arrow on shoplifting charges in Victoria, B.C. Arrow is wanted by the FBI in connection with the 2001 bombings of several logging and cement trucks in Oregon. He has been on the run since being indicted in 2002 and is seeking refugee status in Canada to avoid extradition to the United States.

In January, three Virginia teenagers pleaded guilty to vandalizing fast-food restaurants and damaging construction equipment in 2002 on behalf of ELF. Authorities say they unraveled another ELF plot when they arrested four teenagers in February planning to vandalize SUVs near Richmond.

No one has died as a result of any action claimed by ELF, but authorities are concerned that someone will be injured or killed. "Their goal is to try to maintain this Robin Hood mystique about them. They're not Robin Hood. They are very destructive, and it's just a matter of time before someone dies as a result of their acts," said Rep. Scott McInnis (R-Colo.). McInnis represented the Vail area when a 1998 fire at a ski resort there caused $12 million in damage. ELF has claimed responsibility for the fire. The case remains unsolved.

And when large-scale arson attacks make the nightly news, the general public tends to lump all the environmentalists together instead of separating the mainstream groups from the radicals, McInnis said. "This only hurts the message that legitimate environmental groups are trying to put forward."

Clippinger Chevrolet in West Covina, Calif., was among the auto dealerships in the Los Angeles area where SUVs were vandalized last year. A fire that destroyed a home, bottom center, under construction in San Diego was attributed to the radical environmental group Earth Liberation Front.