WASHINGTON -- On Friday morning, a man matching the description of Russell Eugene Weston Jr., the suspect in the killings at the Capitol, approached a stranger in Lafayette Park, across the street from the White House.

"Millions of people are going to die because of the people in that house," he said, pointing at the president's mansion, according to testimony from an eyewitness that was given Saturday to the Secret Service.

"It's like Nazi Germany," he said. "Millions of people are going to die because of their beliefs."

Less than six hours later, Weston shot up the Capitol, killing two police officers and wounding a visitor, law-enforcement officials said.

If the man in Lafayette Park proves to have been Weston, the brief and odd statements were those of a man who had been formally judged to be mentally ill and a danger to himself and others in 1996, the year he first came to the attention of the Secret Service.

Weston's Secret Service file was only one among thousands kept on people who have threatened the president and other government officials. Though Weston believed that the government was spying on him through a neighbor's television satellite dish, his threats did not result in his being placed under any kind of surveillance by law-enforcement agencies.

The Secret Service knows many such people. "We conduct 200 threat investigations a month," a senior Secret Service agent said, "and they don't all have tinfoil on their heads."

But many are, in fact, mentally ill. A recent study found that 328 people had tried to enter the White House grounds over a three-year period, and that 91 percent were later judged to be schizophrenic, and 66 percent were paranoid schizophrenics, said Dr. Irving Gottesman, a clinic psychologist who has written extensively on the disease.

The delusions voiced by such people are familiar to anyone who has worked at a courthouse, a newspaper or a police precinct. A surprising number of people think the CIA has placed transmitters in their molars, or that the Pentagon is beaming death rays through their televisions, or that the White House is secretly controlling their brains.

"We don't know the root cause of delusions," Gottesman said. "But their content is culturally determined."

Weldon Kennedy, a former deputy director of the FBI, was quick to recall that Timothy McVeigh, who blew up the federal building in Oklahoma City in April 1995, killing 168 people, believed that the Army had implanted a computer chip in his buttocks.

There has never been a finding that McVeigh was insane; in fact, as Kennedy said, the Oklahoma City bombing revealed that of "salt-of-the-earth people, a good portion believe that government itself is evil, and there's much more of that than anyone ever realized." The government's policing such people is a practical impossibility as well as a common paranoid fantasy.

The Secret Service created a new security perimeter around the White House in 1994 after one man sprayed the mansion with gunfire, and another crashed a light plane onto the White House lawn.

It closed off Pennsylvania Avenue in front of the White House, creating a pleasant and patrollable walkway between the mansion and Lafayette Park, where the man who closely resembled Weston on Friday confronted the eyewitness, John Broder, a White House correspondent for The New York Times.

The man was carrying something that resembled an Army ammunition bag, and began the encounter by asking: "Do you know what the storm clouds of war are?"

The Secret Service cannot lock up or look in on the countless numbers of people who could potentially threaten the public representatives of a government they find threatening. But the attack at the Capitol was the latest in a long string of assaults on public figures -- like John Hinckley's shooting of President Reagan in 1981 -- by people who have both a mental illness and a handgun.

Gottesman stressed that he was not attempting to evaluate a man he had never met. But he said he found the accounts of Weston's recent history -- the delusional rage at the government for planting land mines on his half-acre grubstake in Montana and beaming mind-control rays through his neighbor's satellite dish -- "frighteningly familiar."

"It sounds prototypical of the kinds of people who approach the White House," he said. "Those are textbook cases of paranoid schizophrenia -- a preoccupation with systematized delusions, often a theme of a persecution. You are an individual important enough so as to warrant the intervention of these powerful forces. This is the misguided logic of a person suffering from severe mental illness."

The problem for law enforcement, said Kennedy, now the vice president of Guardsmark, a private security firm, is that a tiny but growing fragment of the larger population increasingly holds beliefs that resemble the ravings of a madman.

"This has been out there all along, but it is a growing phenomenon," he said. "The investigation of the Oklahoma City bombing really focused the FBI on the fact that there was a much larger problem here than the FBI had at that point estimated. It existed in more than just isolated pockets. After Oklahoma City we realized that there's a lot more people out there than a few isolated groups that think this way.

"It's a larger part of the society itself" that believes that the government is out to get them, he said, and some of these people have reached "the point where they feel it's necessary to take action."