Updated

This weekend, in Vanishing Freedom II: Who Owns America?, Fox News takes a look at how some forms of environmentalism chip away at individuals' personal and property rights.

Hosted by William LaJeunesse, the special airs Saturday May 19 at 10 p.m. ET/7 p.m. PT and Sunday May 20 at 8 p.m. ET/5 p.m. PT. To find Fox News Channel on your local cable system, click here.

In this, one of several stories from the special, Fox News looks at how environmental extremists are taking their wars to ever-more-extreme lengths.

PORTLAND, Ore. — They think of themselves as the army of Mother Earth, the only soldiers defending the natural world in a guerilla war against corporations and uncaring governments. They are willing to go to extreme lengths for their mission.

Others call them eco-terrorists.

"It's all about anarchy, revolution and terrorism," said private investigator Barry Clausen, an author who has made a career out of infiltrating what he calls "eco-terror" groups.

Using increasingly violent tactics, these extremist groups are waging war against what they see as the enemy — lumber companies, research labs, even car dealerships.

"We can no longer allow the rich to parade around in their armored existence," the Earth Liberation Front said in a statement following an attack on a Portland, Ore., car dealership selling sport-utility vehicles.

In the last few years, the ELF, the most well-known group in the radical fringe, has taken credit for attacks on Michigan State University's Agriculture Hall, a ranger station, a Colorado ski lodge and two lumber companies.

The ELF isn't alone. From the Strawberry Liberation Front to the Anarchist Golfing Association, radical-environmentalist wrecking crews are vandalizing spots across the country.

Last year, a group trampled the Cold Springs, N.Y., cornfield where Nobel Prize-winning genetics research had been done. Turns out that the corn they trampled was entirely natural, not the genetically engineered variety that the group was apparently protesting.

"We've seen it go from demonstrations and protests — which are the American way — and lobbying Congress" to "a large-scale quantity of arsons throughout the country," FBI Special Agent David Szady said.

The washboard and protest sign? They're giving way to the Molotov cocktail and fertilizer bomb.

"When you look at where they buy their literature, it doesn't just include arson. It includes how to manufacture some pretty serious bombs," Clausen said.

And with every spurt of violence, mainstream environmental groups cringe, worrying that a handful of extremists are giving a bad name to a movement that's been gaining strength with its concern over important issues.

But signs don't get results. Bombs do, the ELF says.

"They feel that mainstream tactics used throughout history to further the environmental movement aren't working, and that it's time to take it a bit further," said ELF spokesman Leslie James Pickering.

The definition of the traditional radical Green's target is changing. On Long Island, ELF activists burned down 11 homes in a housing development as a statement against urban sprawl.

"Those were not homes, they are houses, which are part of a development that are threatening the last wild areas in Long Island," Pickering said. "Property is not human, it is not violent to destroy property.

"Anyone, potentially, who is making a profit off the destruction of the natural environment could be a target," he said. "One of the goals is to put the corporation or target out of business immediately."

The ELF routinely sabotages logging equipment, driving metal spikes into trees that can later injure timber cutters. In California in one case, a man nearly died, Clausen said.

"The saw blade hit the spikes, broke the blade," he said. "It wrapped around an employee's body, it cut his jugular vein, tore out twelve teeth and broke jaw. But he did live."

It's difficult in many cases to crack down on groups like the ELF because they often don't have traditional organizations. The spokesman often doesn't know the identity of other group members, while a small core group puts out a free-form call for "direct action" that means any disgruntled teenager with an environmental bent could decide he is working on behalf of ELF or another group.

"They're an underground organization that consists of numerous cells throughout the United States," Szady said.

Currently, the FBI is on guard in case the ELF escalates to letter bombs targeting people it deems enemies to the Earth.

"We don't care what you believe — just don't commit criminal acts to further those acts," Szady said. "As we often say, if you believe the world is flat, don't just go around blowing it up to make it flat."