TERROR IN OKLAHOMA CITY : Tighter Security Carries a Cost : Safety: New constraints on access to public buildings are possible. Experts warn of financial and civil liberty impact in reducing danger.

The kind of high-powered car bomb that ripped through a federal building in Oklahoma City poses an unprecedented challenge to security in the United States and could lead to new measures, including restricting traffic around government facilities, changing how public buildings are constructed and intensifying intelligence efforts to detect possible terrorist plans.

But counterterrorism experts emphasized that any efforts to enhance security will entail significant financial and civil liberty costs and would provide no assurance that future attacks would not simply exploit more vulnerable targets or deploy different weapons. New measures must also be weighed against the openness of U.S. society and the need to provide continued public access to government buildings, they said.

“The fundamental difficulty is that terrorists can attack anywhere or any time they like,” said Bruce Hoffman, director of the study of terrorism and political violence at St. Andrew’s University in Scotland and a former specialist at the RAND Corp. think tank in Santa Monica. “We build a 10-foot high fence and terrorists merely build an 11-foot ladder.”

Nonetheless, the explosion at the nine-story Alfred P. Murrah Federal Building produced reverberations throughout the U.S. government. Security was immediately intensified and the tragedy was expected to prompt consideration of various new steps.

“People should not become fatalistic that there is nothing we can do,” said Larry Johnson, a private security consultant in Bethesda, Md., who previously worked at the CIA and the State Department.

Indeed, other cataclysms have provoked changes in the past. Metal detectors and X-ray machines were installed at airports after a rash of airline bombings in the 1970s. Barriers and safeguards were added at prominent public buildings in Washington amid escalating terrorist threats in the early 1980s.

Several steps could be taken to provide a cushion of safety around federal buildings, experts said. Concrete pillars, which have previously been installed at the White House and Capitol to deter vehicular suicide bombers, could be deployed. Bomb-detecting dogs or electronic devices could be used around the perimeter of such structures. Guards could be posted outside facilities to watch for suspicious or unattended vehicles.

Measures to keep vehicles farther away from buildings, including restricting parking or nearby traffic, may be considered. Experts emphasized that the force of a blast diminishes exponentially when the source is farther from the target; an explosive device two feet from a building will have an impact only one quarter as great as one detonated right next to the structure.

To the extent such attacks are considered a serious risk, future damage might be mitigated through engineering and architectural modifications. After the suicide car bombing of the U.S. Marine barracks in Beirut that killed 241 in 1983, new construction guidelines were adopted for embassies abroad to try to prevent and better withstand such attacks.

In addition to requiring that the structures be set back at least 100 feet from a protective perimeter wall or fence, the mandates called for thick, heavily reinforced concrete walls and small thick windows of plastic and glass that would not shatter easily, said a State Department official.

In contrast, many modern office buildings, such as the 380,000-square foot Oklahoma City structure, which was completed in 1977, were built with large glass windows to maximize natural light and with walls that were not intended to withstand any kind of shock.

Even before Wednesday’s blast, efforts were under way to strengthen government structures. The National Academy of Sciences convened a group of experts last July to prepare a report assessing whether military technologies used to make buildings more blast-resistant could be applied to non-defense structures, a panel member said. The report is expected to be released later this year.

Still, several security specialists stressed the practical restrictions of any such approach.

“If you’re building a structure that would be ideal against a car bomb, you’d build a pyramid,” said Brian Jenkins, a private counter-terrorism specialist who worked on the investigation of the World Trade Center bombing and served on the panel that evaluated embassy security. “Solid construction, sloped walls, no windows and you’d have people working inside underground. But we don’t live in pyramids.”

Although U.S. government buildings may represent appealing symbolic targets for terrorists, experts said that expensive efforts to protect federal buildings could simply result in assaults on less secure places such as schools, shopping malls, airports, subways or crowded city streets.

Moreover, they noted that this week’s bomb was an unsophisticated, if powerful, device; in the future, more advanced bombs could wreak even greater damage from greater distances, or biological or chemical weapons could be unleashed to contaminate the air in an enclosed public place or the water supply of a major city.

Hence, most specialists said, the anti-terrorism effort should focus on intelligence-gathering to identify conspiracies as they unfold and prevent such violence in the first place. Some said this could lead to tensions over constraints on such domestic activities that are intended to protect individual rights.

Times staff writer Robin Wright contributed to this story.