WASHINGTON — An elderly man enters a crowded museum carrying a rifle and begins shooting. A young man in Arkansas pulls the trigger outside a military recruiting office. Another man opens fire in a Kansas church.

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" attacker, a killer who, authorities say, is harder to head off than an attack 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 finding 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 if someone will put down a laptop and pick up a gun.

When investigating a suspected 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 an ideological killer acting alone rarely tells anyone what he's planning, let alone when or how. That makes it hard for authorities to determine who plans to commit a criminal act in furtherance of a perceived cause.

To try to deal with that, the FBI uses what it calls "tripwires" — programs that seek tips from businesses whenever someone buys significant amounts of materials that can be used to make explosives, or large numbers of weapons or ammunition.

Such precautions seem to have paid off 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 Obama. The actions by that man, who relatives said suffers from mental illness, triggered an investigation and he was eventually arrested.

In Washington, white supremacist von Brunn used a vintage rifle from the early 20th century, something that would not have triggered such tripwires. 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 a 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."