PHOENIX (AP) _ They've threatened to blow up power plants, government buildings, even the Hoover Dam. Some have held military exercises in the desert, firing automatic weapons and setting off bombs.

With its wide-open desert spaces and tolerance for guns, Arizona has long been a breeding ground for self-styled militia members. The indictment Monday of 12 members of a group called the ``Viper Militia'' was just one in a series of cases involving paramilitary activity dating back to the 1970s.

Arizona is ``in the top five or 10 states'' for militia activity, said Bryan Levin, an expert on the movement and assistant professor of criminal justice at Stockton College in New Jersey. He estimated as many as 30 militia groups have surfaced in the state.

``These types of movements are particularly attractive to the West, not just Arizona,'' Levin said. ``Part of it is the issues _ land use, gun rights _ things that may not be as important in some other parts of the country.''

Neither Levin nor David Rosenberg, who tracks militia groups for the Anti-Defamation League B'nai B'rith in New York, had heard of the ``Viper Militia,'' the group accused in a federal indictment released Monday of plotting to blow up government buildings in the Phoenix area.

There has been speculation that the group took its name from ``OPLAN American Viper,'' a plan for overthrowing the government that has been distributed anonymously at gun shows and other gatherings around the country.

The 68-page document includes instructions for attacking targets from water supplies and rail and communications lines, to airfields, fuel depots and ammunition dumps.

It warns of a plot by an unidentified world power to ``disarm and enslave the American people.''

``The whole idea behind it is the violent overthrow of the government,'' said Morris Dees, who tracks hate groups for the Southern Poverty Law Center in Montgomery, Ala. ``It goes into intricate detail about how to do that.''

Arizona has been home to militias since the 1970s, when Robert Matthews began conducting paramilitary maneuvers in the desert near Cave Creek. Matthews eventually formed The Order, a violent, Idaho-based group accused of bank robberies and murder. Matthews was killed in a 1984 shootout with FBI agents.

In 1987, three members of the Arizona Patriots were imprisoned for plotting an armored car heist to bankroll a paramilitary complex near the northwestern Arizona town of Kingman. Investigators later learned the group planned to bomb government installations including the Hoover Dam.

But little media attention was paid to the gun-toting, camouflage-clad ``patriots'' in Arizona until the Oklahoma City bombing.

Timothy McVeigh, the lead suspect, was quickly traced to Kingman, where he sometimes stayed with Army buddy Michael Fortier.

Fortier has since pleaded guilty to knowing about the bombing plot but concealing it from authorities. He is expected to testify against McVeigh and co-defendant Terry Nichols.

There was also fear of a militia connection when the tracks under Amtrak's Sunset Limited were sabotaged last October along a desolate stretch of desert, plunging the train into a dry gulch. A crew member was killed and nearly 80 people were injured.

No one has been charged in the derailment. The FBI said today it was investigating whether members of the Viper Militia was involved.

An anti-government letter found at the wreck site, signed by ``Sons of the Gestapo,'' included anti-government references to Waco and Ruby Ridge.

The 1992 Ruby Ridge confrontation led to the deaths of a federal marshal and the wife and son of white separatist Randy Weaver. Near Waco, Texas, more than 80 people were killed in a 1993 fire that destroyed the Branch Davidian compound after a 51-day standoff with federal agents.

Arizona also is attractive to militia groups because the state's gun laws are among the most liberal in the nation. State law allows virtually anyone who is not a convicted felon to carry a loaded gun, as long as it is not concealed. In addition, 50,000 people have obtained concealed weapon permits.

``Guns are part of the landscape out there,'' Rosenberg said. ``Of course, most people don't use them to attack federal institutions, but it is that atmosphere that makes it easier for those who do want to join these militant, anti-government groups.''

Arizona's terrain _ endless miles of open desert and dense mountain forests _ also is perfect for paramilitary maneuvers, Levin said.

``In northern New Jersey,'' he said, ``it would be quite hard to go about blowing things up without attracting the attention of your neighbors.''