Three chilling, unconnected slayings in less than two weeks. One gunman was a white supremacist, one a militant Muslim, one a fervent foe of abortion.

Each suspect had a history that suggested trouble. Each apparently was driven to act by beliefs considered by some as extreme. Each shooter fits the description of a "lone wolf" terrorist, a killer whose attack, authorities say, is harder to head off than if planned by a trained terrorist network.

"It could be anyone. It could be the guy next door, living in the basement of his mother's place, on the Internet just building himself up with hate, building himself up to a boiling point and finally using what he's learned," said John Perren, head of the counterterrorism branch at the FBI's Washington field office.

Perren described the difficulty of hunting a lone wolf suspect in an interview with The Associated Press just two days before white supremacist James von Brunn allegedly shot and killed a guard at the U.S. Holocaust Memorial Museum.

"The lone wolf is what concerns the Washington field office, what concerns the FBI the most," he said.

Von Brunn had a criminal record and a Web site preaching virulent racism. Yet authorities say it is difficult to predict when someone will put down a laptop and pick up a gun.

When investigating a terrorist network, FBI agents can often access e-mails, phone records and documents to build a case. In some cases, they can develop informants to penetrate the group and provide intelligence.

But a killer acting alone rarely tells anyone what he's planning, let alone when or how. That makes it hard for authorities to determine who is prepared to commit a criminal act in furtherance of a perceived cause.

Trying to counter that threat, the FBI has created what it calls "tripwires." These are programs that seek tips from businesses whenever someone buys significant amounts of materials that can be used to make explosives, or large amounts of weapons or ammunition.

Such precautions seem to have worked in the case of a man who cleaned out his savings account in Utah and told the bank teller he was on a mission to kill President Barack Obama. The man, who relatives said suffers from mental illness, triggered a criminal investigation and he was eventually arrested.

In Washington, the white supremacist von Brunn apparently skirted such tripwires by using a vintage rifle from the early 20th century. With that single, small-caliber gun, a museum security guard was killed before other guards opened fire and disabled von Brunn.

The Southern Poverty Law Center, which tracks white supremacists, says the number of hate groups in the United States has risen 54 percent since 2000, fueled by opposition to Hispanic immigration and, more recently, by the election of the nation's first black president and the economic downturn.

"Today the vast majority of domestic terrorist attacks are in fact lone wolf or so-called leaderless resistance attacks," said the center's Mark Potok. "There are very few ways to prevent them ... short of assigning a police officer to every person in America."

The number of angry white men in America is getting larger, said Chip Berlet, senior analyst with Political Research Associates in Somerville, Massachusetts, a think tank that studies right-wing extremists.

In particular, the heterosexual, white, Christian men in America feel they've been pushed out of the way, Berlet said. Attacking the Holocaust Museum is obvious, he said, because white supremacists blame Jews for the advancement of black people.

"The idea that blacks are put in positions of power by crafty Jews is central to their conspiracy theory," Berlet said.

Other experts contend the white supremacist climate has not changed much over the years.

Charlie Allen, the former top intelligence officer at the Department of Homeland Security, said such hatred has been embedded in small parts of American communities. Under Allen's leadership, the department created an analysis branch that looked at extremist groups across the country.

A department assessment of domestic extremism found about 2,400 white supremacist Web sites; 72 blogs; 30 mailing lists; 213 user groups and clubs; and 25 online racist video games.

The agency says white supremacist organizations rarely publicly call for attacks.

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