New York ? The men who showed up at John Young’s door were courteous and professional. Also intimidating. They were, after all, from the FBI.

“Just a visit by the FBI has overtones,” said Young, a 68-year-old activist who says the government has been monitoring a Web site he runs ever since the agents visited late last year. “Whether you’ve done anything wrong or not, you think, ‘Oh no.”‘

With the Republican National Convention less than two weeks away, federal agents and city police are keeping tabs on activists and others they believe might try to cause trouble. They are making unannounced visits to people’s homes, conducting interviews and monitoring Web sites and meetings.

The effort has been overshadowed by far-reaching counterterrorism measures planned for the Aug. 30-Sept. 2 event. Officials will not discuss it on the record, other than to say investigators always act within the law.

“We’re not engaging in surveillance of groups or individuals without legal predication,” said Jim Margolin, a spokesman for the FBI’s New York office.

Ann Roman, a Secret Service spokeswoman, said its agents expected to respond to an increase in possible domestic threats against President Bush and other dignitaries as the convention at Madison Square Garden nears.

“How we do that specifically, I’m not going to go into,” Roman said.

According to three law-enforcement sources, federal agents in New York have begun interviewing people they believe might know about plots to sow mayhem at the convention, and have used surveillance against possible suspects.

The intelligence unit of the New York Police Department has been watching Web sites run by self-described anarchists. It also has sent young, scruffy-looking officers posing as activists to protest-organizing meetings, said one high-ranking law enforcement source, who spoke on condition of anonymity.

In recent weeks, several people linked to anarchist groups in Colorado, Kansas, Missouri and elsewhere have reported being “harassed” by federal agents about the convention.

In late July, the FBI questioned at least two members of a Lawrence, Kan., anarchist group and confirmed agents were checking out a tip that a group planned to firebomb media vehicles at the Democratic National Convention in Boston.

“It just seems like it’s information gathering. It’s intelligence gathering,” group member David Strano then told the Journal-World. “It’s probably useful to them for something.”

Bob Herndon, an FBI spokesman in Kansas City, Mo., said the agents were merely trying to find out whether there was any truth to a tip received by the Joint Terrorism Task Force, which provided security for the convention.

“What we’re investigating are specific threats,” Herndon said. “We’re not investigating ideas per se.”