WASHINGTON - On a March day in 1981, John Hinckley Jr. stood outside the Washington Hilton Hotel, and for the love of a movie star, fired a bullet from a handgun into President Ronald Reagan's chest as the nation's leader left a speech in Washington. On a June night in 1968, Sirhan Sirhan appeared in a crowd at a Los Angeles rally for Sen. Robert F. Kennedy and shot him to death. In 1971, Arthur Bremer lunged forward in a crowd listening to a political speech, fired his revolver and left Alabama Gov. George Wallace paralyzed from the waist down.

Russell Eugene Weston Jr., a paranoid schizophrenic who police say burst through a checkpoint at the U.S. Capitol Friday and gunned down two federal police officers with his handgun, became only the latest.

A long line of individuals, without orders from some shadowy figure above, or a mandate from an underground organization, have taken it upon themselves to commmit deadly acts.

The nation is still threatened by underground militias, violence-prone hate groups and shadowy terrorist cells. But for decades it has been the lone attacker, often mentally deranged and spurred on by a silent grudge or searing hatred, who has posed the greatest threat to public figures, forensic psychologists say. Because these people operate in isolation, experts say, it is nearly impossible to predict where - or when - they might strike.

"The government keeps beating the drums about foreign terrorism and militias," says Thomas Ressler, a former FBI profiler. "But these people are the ones who are the most dangerous. They are the lone gunmen and they are the ones who come out of the blue. . . . You can't prevent against the unknown."

The ranks of potential attackers are not thin. Secret service officials have thousands and thousands of them in their files. Some are reported by other law agencies, some because they have written something that seemed threatening, others because someone, somewhere, reported them as dangerous. Each has been interviewed and rated according to the level of danger agents think they pose.

More than half, one agent says, have mental problems.

Weston, 41, who claimed President Clinton was "out to get him," was charged Saturday with murdering two Capitol police office officers, Jacob Chestnut and John Gibson.

Weston had made the list of potential attackers before. Twice in 1996, he reportedly made verbal threats against Clinton. After each report, agents visited him, according to the procedure followed with each of the estimated 300 threats made against the president each month. They checked his criminal background and found no trace of violent criminal acts, an aspect of the background check that raises the threat of danger. They noted that he had lately been in a mental hospital.

Without any legal reason to detain him, to seek a wiretap, or to conduct surveillance, they simply put him in the computer with thousands like him. That he was not recognized as a threat is proof of the problem.

It is "like watching a flake in a snow storm," says Jack Levin, a criminology professor at Northeastern University, who has studied lone attackers for 20 years.

"There are simply too many," he says. "We can identify people who are likely to cause trouble for others but they share these signs with hundreds of thousands of people who don't harm anyone. So what do we do? I'm afraid this is one of those cases where there's no answer."

Isolation a factor

Such people have played a role in some of the most infamous crimes of the century. In 1901, an anarchist named Leon Czolgosz assassinated President William McKinley. Sixty-two years later, Lee Harvey Oswald fired into a motorcade and assassinated President John F. Kennedy. Five years later, James Earl Ray assassinated Dr. Martin Luther King Jr., although his role as the lone gunman is questioned. And Theodore J. Kaczynski, the Unabomber, terrorized the nation for 18 years, building bombs that killed three people before he was finally arrested in 1996.

"A lot of these people have been found to be socially isolated or social outcasts," said California forensic psychologist Reid Meloy. "That isolation incubates anger and resentment over time and their behavior takes on a paranoid quality."

In an article written a decade ago for the Journal of Forensic Sciences, Park Dietz, a forensic psychiatrist, sounded an alarm about lone attackers: "The extent to which many subjects focus their attention on multiple public figures, including both entertainers and political leaders, calls for new approaches in the protection of public figures."

Dietz, who studied people who wrote threatening letters to members of Congress and others in the public eye, points out that political leaders are not alone as targets of such people.

Mark David Chapman, an obsessed fan, gunned down former Beatle John Lennon as the musician walked out of is New York apartment.

Many motivations

Unlike Timothy McVeigh, the mastermind behind the Oklahoma City bombing, Weston appears to have had no murderous plan drawn up in advance. But Weston fit the profile of a public shooter almost perfectly, Ressler says.

Such assailants tend to be white males, aged 37 to 43, and in midst of a family squabble or break up of a relationship. Often they are diagnosed as paranoid, schizophrenic, or both, like Weston. People with such disorders are often preoccupied with a delusion and may be anxious, angry or even violent, according mental health guides.

In the case of lone assailants, says Ressler, "normally, you will find diaries, writings, scrapbooks that will focus on paranoid ideals."

Government officials familiar with the ongoing investigation say letters were found in Weston's truck that indicated Clinton and the government were out to get him.

Tracking lone suspects is especially difficult, authorities say, because their paranoia often keeps them from joining established groups or associations. But in their efforts to identify and understand lone attackers federal officials have asked psychologists to help.

Two months ago, the Secret Service published the findings of an ongoing study that looked at 83 people whose threatening behavior or violent acts against public figures date back to 1949. About half those studied were found to have displayed evidence of delusional or psychotic behavior during their lives. In his most recent book, Psychology of Stalking, forensic psychologist Reid Meloy wrote that "the common thread that emerges is the government as the enemy. It is indeed a sobering and serious phenomenon that we are seeing in this country."

Creating a working profile of the assassins and other "attackers" has proven difficult because of the myriad factors that motivate them. But the Secret Service has been able to identify eight "major motivators." They are:

A desire for notoriety or fame.
An attempt to avenge a perceived wrong.
A desire to develop a special relationship with the target.
Financial gain.
A desire to bring about political change.
To cast attention on a personal or public problem.
To bring an end to personal pain by risking death.
To save the country or the world.

Brian Levin, a professor at Stockton College in Pomona, N.J., who studies exremists, notes that a number of factors can push a psychologically troubled person over the edge, including a lack of treatment, the loss of a job, an altercation with a family member or another disappointment.

Weston's father says he kicked his son out of his house in Valmeyer, Ill., after he killed as many as 16 cats with a shotgun. By Friday, Weston had made his way from a Montana cabin to Washington D.C.

He was not high on the list of people the Secret Service worried about, despite threats in 1996 against the president. He was in the category of people that agents would check on if the president had headed to Montana.

He had made no more threats that agents knew about after being investigated for those earlier remarks, and he had no record of violence.

Monitoring him without probable cause would have been in violation of Weston's civil rights. And shortly after the federal interviews, Weston was committed to a mental institution where he received treatment and got a clean assessment from doctors.

Friday, Weston was in Washington's Lafayette Park, shouting at the White House, shortly before the shooting spree. But shouting or protesting outside the White House is not a cause for detaining anyone.

Because visitors to the White House are pre-screened, as are those to the Pentagon or to the Central Intelligence Agency, he could not have gotten into any of those facilities without triggering alarms within the computer tracking system.

But there is no pre-screening mechanism at the Capitol beyond the metal detectors at the entrances. And Weston was already inside - and shooting - when guards realized that he was carrying a weapon.

Options limited

There was nothing the Secret Service could have done to prevent last week's tragedy, says Ron Noble, a former undersecretary of the Treasury who was in charge of the White House security assessment in 1994.

The Secret Service "doesn't have the manpower, responsibility or right to follow people who are no longer a threat and haven't committed a criminal act," he says. "He was referred to a mental hospital. He was released. He hadn't made a threat after he was released. The next time he acted, he acted with a gun and he killed two officers."

By Charisse Jones and Gary Fields, USA TODAY

Contributing: Kevin Johnson