Norman Mayer, the 66-year old anti-nuclear activist who died after he threatened to blow up the Washington Monument during a 10-hour siege yesterday, was described by friends and acquaintances as a loner and drifter who has waged a crusade in recent years to stop what he called "this madness" -- the spread of nuclear weapons.

Friends and acquaintances of Mayer reported being interviewed yesterday by FBI agents and police who fanned out throughout Washington and other parts of the country.

Law enforcement sources said that investigators were checking reports that Mayer had attempted to purchase explosives in several different places, including Frederick, Md., Fairfax County and Hazard, Ky., and that he had told others he was planning to do something dramatic.

James Grollman, a 19-year-old George Washington University student, said in an interview yesterday that he talked to a man he believes was Mayer three weeks ago in front of the White House and "he was upset because the world is not listening. He said he was going to do something soon. I asked him what it was. He said, 'The whole world is going to find out.' "

Friends and acquaintances said Mayer was consumed with a passionate belief that "doomsday" is at hand. His solitary obsession took him first to Florida college campuses, where he was twice arrested in 1979 while passing out strident antinuclear leaflets, according to Florida court records that showed he was subsequently acquitted.

In recent months he made unsuccessful attempts to enlist the aid of Washington-based antinuclear groups, and spent hour after hour parading back and forth in front of the White House, clutching a wooden sign that bore his message of impending death and destruction.

Mayer, described by those who met him as having a deep Florida tan and the build of a much younger man, had been staying on and off for the last eight months in a bare-walled, $29.60-a-night room at the Downtown Motel at 1345 New York Ave. NE, according to Robert Elliott, one of the motel's owners.

Motel employes described Mayer as a courteous, quiet man whom they nicknamed "Pops." He checked out yesterday morning, according to desk clerk Julia Antrum. "He said he might be back," she recalled. "This never seemed like something 'Pops' would do."

Motel officials said Mayer paid his bill like clockwork every two days with $100 bills. Charles Sims, a District sign maker who became acquainted with Mayer when he made two signs for him, said Mayer told him he had spent $30,000 in the last year fighting nuclear weapons.

Relatives and friends said Mayer, whom they described as unmarried, had lived in Miami Beach and the Virgin Islands in the last several years, working at a variety of jobs in hotels and other businesses.

Mayer's brother, Aubrey, 67, of Los Alamitos, Calif., said he was called almost immediately by the FBI yesterday, and agents at one point were considering having him talk to his brother by telephone. "I don't think it would do any good. If he's made up his mind about something, he's made it up," Aubrey Mayer said. "He never listens to me anyway. The only person who might have been able to talk him out of it was our mother, and she's dead."

He said the family lived in New Orleans while they were growing up and that his brother had been in and out of trouble over the years.

Law enforcement sources said that Mayer, who was born in El Paso, Tex., and served in the Navy during World War II, had an arrest record that includes a 1949 charge in Los Angeles related to a prowling incident, a 1957 assault and battery charge in Hialeah, Fla., and a 1976 drug arrest in Hong Kong.

His sister-in-law, Beth Mayer, said he had worked, among other things, on an oil drilling rig, as a mechanic and as a toolmaker.

The sister-in-law said she and her husband rarely heard from Mayer, but that he would occasionally mail them antinuclear literature. In an interview hours before Mayer's death, she said that when she heard his name on the radio yesterday, "I laughed. I thought it could be some kind of nutty thing he might try like so many of the others."

She and others who knew Mayer said they found it hard to believe that he would resort to violence.

The 1979 truck parked at the monument and registered to Mayer lists the Miami Beach address of Jack's Foreign Auto Service, a repair shop that Mayer also used as a mailing address on his leaflets.

Kay Bauer, whose husband Jack owns the shop, said they had kept Mayer's mail since April, the last time they saw him, until the FBI confiscated it yesterday.

In recent months Mayer received a cool reception when he tried to press his personal brand of antinuclear politics on several Washington peace and antinuclear organizations, including Ground Zero, SANE, and The Arms Control Association.

"I would have to characterize him as a very angry man," said Ground Zero press spokesman Ellis Woodward, who said he talked to Mayer for about a half-hour when he showed up in the group's downtown office in August.

Woodward and officials of other similar groups said that they politely dismissed Mayer as one of the many eccentric people who often appear in the offices of political-action groups.

"I tried to reason with him and use logic, but that didn't seem to work at all," said George Stephanopoulos, 21, a staff member at the Arms Control Association. Stephanopoulos said he frequently saw Mayer passing out leaflets on Dupont Circle near his office and spotted him parading in front of the White House within the last two weeks.

Stephanopoulos said Mayer was upset that the antinuclear groups were not moving fast enough to save the world from a nuclear holocaust.

"We are in a failed civilization daily on the verge of annihilation and all the world's establishments are responsible," declared a Mayer leaflet that also included drawings of mushroom clouds and human skulls and bore the legend: "Ban nuclear weapons or have a nice Doomsday."