Police can block off streets, tow suspicious cars, empty garages and erect reinforced concrete barriers, but in the end there are always too many targets to protect. The best defense against terrorists, experts said yesterday, is to find them before they can do any damage.

"The reality is that it's almost impossible to secure any building," especially when there is parking or traffic on nearby streets, New York City Police Commissioner William Bratton said. "The best offense in a lot of this is very good intelligence about these characters."

Jittery authorities heightened security measures at 8,200 federal buildings around the country yesterday in the aftermath of the car bombing Wednesday that gutted a federal office building in downtown Oklahoma City, killing at least 40 people, with hundreds still missing.

For the second day in a row, bomb threats closed federal office buildings. Yesterday facilities in 11 cities, including Philadelphia, Atlanta, Houston, Nashville, and Oakland, Calif., closed. A bomb threat shut down the Agriculture Department complex on Washington's Mall at 5 p.m.

The General Services Administration decreed a "moderate security alert," requiring both federal workers and visitors to present photo IDs as they entered buildings, and ordering vehicles, packages and briefcases searched. GSA said security personnel were paying "particular attention" to parking lots, garages and parking spaces adjacent to federal buildings.

House Permanent Select Intelligence Committee Chairman Larry Combest (R-Tex.) said Congress would consider limiting public parking in federal garages, prohibiting curbside parking around federal buildings and other "reasonable changes" to make federal installations less of a target.

GSA said it was reassessing security at 98 federal child care centers nationwide, and in "a significant number of cases" had stationed uniformed personnel to provide additional security. The bombed building's second-floor day care center was obliterated in the explosion.

"Given today's climate, I wouldn't put a day-care center anywhere near" federal law enforcement offices, said Tom Doyle, president of Aegis Security Associates and a retired Secret Service agent. "I would rethink putting kids near agencies with controversy attached to them."

In Washington, Secret Service bomb sniffer dogs screened cars along Pennsylvania Avenue but the White House, FBI headquarters, and the monuments on the Mall remained open to tourists.

Heightened security, Park Police spokesman Maj. Robert Hines said, "is something we do automatically in these situations." But, he added, "this is a fine line. This is the tourist season, and we have visitors from around the United States and around the world. We don't want them to feel they are in a police state."

But for terrorism experts inside and outside government, what to do today was not nearly as important as finding a way to avoid another Oklahoma City, and here the consensus was clear: Make the obvious defensive moves, but don't rely exclusively on them, because they won't work.

"For three to six months after the World Trade Center bombing, they really paid attention to security in New York," said Mike Ackerman, managing director of the Miami-based Ackerman Group, an international security consulting firm. "The problem is that after nothing happens, you tend to lose focus. In some ways terrorists can almost predict when we're going to lose focus."

Worse, said Yigal Carmon, a former terrorism adviser to Israeli Prime Minister Yitzhak Rabin, the United States simply cannot protect every potential target.

"Only a small country like Israel can build its security on a defensive approach," Carmon told Washington Post correspondent Barton Gellman in Jerusalem. "Let's take one element, air transportation. We have one airline. We have 1 1/2 airfields. We can put resources in manpower and spending to protect that.

"There is no way that a superstate like the United States, with its scores of airlines, with its hundreds of international airfields, can protect all these," Carmon said.

To gain an edge, the experts agreed, the United States needs to collect better intelligence, take better control of its borders, and monitor foreign residents and perhaps even its own citizens.

"We have to refocus our intelligence efforts," Ackerman said. Given the United States' porous borders, "People come in . . . for whatever reason and never leave, and we don't know they're here. We don't know who's in the country."

"We're open and easy to hit," he added.

With the FBI's announcement yesterday that it was searching for "two white males," one with a light-brown crew cut and the other with a tattoo, however, the presumption that the Oklahoma City bombers were aliens has lost strength.

In this context, the emphasis shifts to domestic intelligence gathering, the experts said, a ticklish matter that treads on constitutional guarantees and inhibits access to public buildings and public figures that Americans take for granted.

Carmon said guidelines should be loosened to allow the Justice Department to gather intelligence on extremist groups without prior evidence of criminal activity.

"You need to build a database to relate to this threat the way one relates to threats abroad," he said.

The United States also should remove the sweeping constitutional protection, under freedom of religion, for the "preaching of violence," Carmon added.

"The western mentality is, if someone says something, who cares? In Islam, and also in Judaism, religion conducts your life. . . . The believers care, the fanatic listeners care, and act upon it."

At the very least, however, the nation needs to reexamine its commitment to a free and open society in light of continuing terrorist threats. "We have to strike the balance between security and free access," Capitol Police spokesman Sgt. Daniel R. Nichols said.

"The Capitol complex is open 24 hours a day," Nichols said.

"People can walk near our buildings any time of the day. People wishing to enter a building to see a member of Congress don't have to have an appointment or state a reason why they are visiting a member."

At least not yet.

Staff writers Barton Gellman in Jerusalem and Ruben Castaneda and Walter Pincus in Washington contributed to this report.