Federal agents and city police are keeping tabs on people they say might try to cause trouble at the Republican National Convention (search), questioning activists, making unannounced visits and monitoring Web sites and meetings.

The law enforcement effort has been going on quietly, overshadowed in public by talk of counterterrorism measures planned for the Aug. 30-Sept. 2 event.

"We're not engaging in surveillance of groups or individuals without legal predication," said Jim Margolin, spokesman for the New York office of the FBI.

Ann Roman, a spokeswoman for the Secret Service (search), said agents expect to respond to an increase in possible domestic threats against President Bush and other dignitaries as the convention at Madison Square Garden nears. The Secret Service is also playing a lead role in planning convention security.

"How we do that specifically, I'm not going to go into," she said.

Law enforcement sources said that in recent weeks, federal agents have begun interviewing people in the New York City area they believe might know about any plots to cause mayhem at the convention, and have used surveillance against possible suspects.

The intelligence unit of the New York Police Department has been closely monitoring Web sites run by self-described anarchists. It also has sought to infiltrate protest groups with young, scruffy-looking officers posing as activists.