For nearly seven years, the nation has turned its terror focus on Al Qaeda and the hunt for Usama bin Laden. But there is a domestic terror threat that federal officials still consider priority No. 1 — eco-terrorism.

The torching of luxury homes in the swank Seattle suburb of Woodinville earlier this month served as a reminder that the decades-long war with militant environmentalists on American soil has not ended.

"It remains what we would probably consider the No. 1 domestic terrorism threat, because they have successfully continued to conduct different types of attacks in and around the country," said FBI Special Agent Richard Kolko.

The FBI defines eco-terrorism "as the use or threatened use of violence of a criminal nature against innocent victims or property by an environmentally oriented, subnational group for environmental-political reasons, or aimed at an audience beyond the target, often of a symbolic nature."

For years, officials have battled against members of shadowy groups such as the Earth Liberation Front and its brother-in-arms, the Animal Liberation Front. Law enforcement has made strides prosecuting cells, but it's been unable to end the arsons that have plagued developments encroaching on rural lands in the West.

FBI estimates place damages from these attacks at well over $100 million. So far, no one has been killed.

It's a problem that's unlikely to go away.

"Every time a fire breaks out and somebody takes a spray can and writes 'ELF' or 'ALF' on there, then everybody gets all excited that 'Oh this movement has started back up,'" said Bob Holland, a retired arson investigator. "The movement never really left."

Fighting for Nature

The Earth Liberation Front rose to infamy in the late 1990s for a series of arsons in the Pacific Northwest targeting industries, such as logging, that the eco-terrorists perceived as a threat to nature.

"Generally speaking, the Earth Liberation folks are motivated by a deep kind of affective connection to nature that many of them would characterize as spiritual or religious," said Bron Taylor, a professor of religion and nature at the University of Florida. "They believe that the human species is perpetrating a war on nature and that those who are connected to nature and belong to it have a right to defend themselves."

Members who carry out attacks in the name of nature tend to be of college age and well educated, and typically have an out-of-town recruiter who lures them into the act of crime, said Ron Arnold, the executive vice president of the Center for the Defense of Free Enterprise.

There are exceptions to the rule, Kolko said. Some members are in their 30s, 40s and 50s.

These eco-terrorists often operate alone or in small groups, making them extremely difficult to penetrate. They target structures they feel are infringing on nature, leaving low-tech detonators that allow the fires to start after they've left.

In 2006, a nine-year federal sting dubbed "Operation Backfire" was able to dissolve a cell responsible for 20 acts of arson in five Western states over five years.

That cell, dubbed "The Family," caused more than $40 million in damage and included attacks on a meat company in Eugene, Ore., a ski resort in Vail, Colo., and the torching of SUVs in Oregon.

The latest Family member convicted, Briana Waters, was found guilty of arson on March 6 in Tacoma, Wash., for her role as a lookout in a 2001 fire that destroyed the Center for Urban Horticulture in Seattle, causing more than $2 million in damages.

"It's a leaderless ideology that can exist for a long time," said Holland, who worked on Operation Backfire. "You take out a cell like we did in Backfire — that doesn't stop like-minded individuals around the country from perpetuating the ideology of the ELF and ALF movement."

Elves in the Night

The perpetrators of the March 3 fires on the Seattle Street of Dreams left their mark, investigators said, with signs that read, "ELF" and "McMansions in RCDs r not green," a reference to rural cluster developments or residential subdivisions, along with an estimated $7 million in damages.

The homes had been built near the headwaters of Bear Creek, which is home to endangered chinook salmon. Opponents of the development had questioned whether the luxury homes could pollute the creek and an aquifer that is a source of drinking water, and whether enough was done to protect nearby wetlands.

In the past, ELF members have used everything from milk jugs to electrical ignition devices to set their blazes, Holland said.

Officials for the fire earlier this month said no explosive devices were found amid the remains of the houses. The Building Industry Association of Washington and the FBI were offering a $100,000 reward for information leading to the arrest and conviction of those responsible.

Finding the culprit in eco-terrorism often requires someone within the organization turning against his or her fellow elves.

"In Operation Backfire, we saw a sophistication that we've never seen in any type of a radical organization, and frankly, if somebody hadn't turned, we wouldn't have enjoyed the success we've had with that," Holland said.

Trying to predict where or when they'll strike next becomes a guessing game, experts said.

"You don't know what's the hot topic in the minds of the potential perpetrators today — you can look on the Web and find out all kinds of things," Arnold said, noting that "it's very difficult to generalize because there are so many threads in the tapestry of environmentally inspired crimes."

The FBI currently has 180 ongoing eco-terror investigations and over the last several years has tied them to some 1,800 criminal acts, Kolko said.

Despite the gains law enforcement has made, it just takes one person to reignite the movement, Holland said.

"There's no way to know or gauge how many people are actually sympathetic to that ideology and will continue to perpetuate it through acts of arson and other violence," he said.

The Associated Press contributed to this report.