Probe of N.Y. Bombing Focuses on Terrorists : Disaster: The possibility of a lone attacker is downplayed. Security is tightened at office buildings.

Major office buildings here adopted stringent security measures Sunday as investigators focused on the likelihood that the gigantic explosion at the World Trade Center was the work of political or drug terrorists.

James Fox, the head of the FBI’s field office in New York, minimized speculation a deadly bomb was planted by a disgruntled person. He said the lone-bomber theory “was definitely on the back burner.”

“It certainly could be anyone. . . . You have some drug dealers who are upset at the U.S. government,” he added.

A jittery city prepared to go back to work today under security precautions like those imposed during the Persian Gulf War. The measures were adopted in the wake of official confirmation from the FBI on Sunday that the basement garage of the trade center had been devastated by a bomb during Friday’s lunch hour.

“We are prepared to say at this time it definitely was a bomb,” said Fox. “ . . . Beyond a doubt, it was a bomb.”

Federal investigators said there was no indication of more than one bomb blast, but they have still not been able to reach the actual site of the explosion--and may not get to that area of the building until early in the week.

The crime scene was described as extremely dangerous, and investigators said they were wary of an additional tragedy inside the garage due to structural failure. Shattered by the blast, slabs of concrete, 12 and 14 inches thick, dangled from parts of the gaping hole that blew through five levels of the center’s basement.

“We’ve got some precipitous overhanging slabs down there,” said Charles Maikish, director of the trade center.

Five people were killed and more than 1,000 injured in Friday’s blast. On Sunday, 40 people remained hospitalized. It was believed at least one person might still be buried in the rubble. One man who had been listed as missing turned up safe, unaware that authorities were looking for him.

FBI Director William S. Sessions, appearing on ABC-TV’s “This Week With David Brinkley,” said findings of nitrate in samples of air and rubble at the blast site indicated that the device was likely to have included dynamite or a fertilizer-based explosive.

Investigators, however, did not rule out the possibility that a combination of devices--including plastic explosives--could have been used. Bomb experts at the scene were searching for chemical “markers” characteristic of plastic explosives.

New York City’s Police Commissioner Raymond Kelly asked businesses to adopt the Gulf War security measures that require, in some cases, package searches and tighter identification of employees and visitors.

He scheduled a meeting with industrial security experts for today as the twin 110-story towers of the World Trade Center remained closed, causing serious personal and economic disruptions.

But Richard C. Leone, chairman of the Port Authority of New York and New Jersey, which operates the trade center complex, sought to minimize concerns about the economic impact of the blast.

“Most businesses are adaptable. . . . We don’t want people to think this will be an economic catastrophe to the region or the nation,” he said.

The investigation into what may be the biggest terrorist attack in U.S. history began just 22 minutes after the blast at 12:18 p.m. Friday.

A New York City Police Department detective in a blue suit hurried into an office building across the street from the smoking and rubble-strewn trade center garage and asked to use a private phone line. He displayed his gold shield for building superintendent Joseph Clark.

The detective, who did not give his name, reported over the phone that two bombs had gone off at the World Trade Center, that there were dead bodies and that he needed Polaroid camera film right away.

Forty-eight hours later, experts were still confined to the fringes of the site because of the structural instability. But sifting the rubble will be only one aspect of the worldwide probe.

The FBI has assessed its computerized data base dating back to the Arab terrorist attack on Israeli athletes at the Olympics in Munich, Germany, two decades ago. Terrorist groups are categorized by types of explosives, detonation devices, methods of delivery and demands. Experts also have charted the tendencies of certain groups to take credit for attacks--or not.

One U.S. counterterrorism official noted, for example, that car bombs--the suspected method of delivery at the trade center garage--are most commonly used by Colombian drug dealers, Irish Republican Army extremists and assorted Mideast terrorists. The expert said that car bombs “are not the standard m.o.” (method of operation) of Yugoslav factions.

Some sophisticated terrorists work hard to disguise their methods, experts said Sunday. For example, simple, readily available materials will be used to build a bomb because these common ingredients are harder to trace.

A Japanese Red Army terrorist who was apprehended with bombs in his car at a rest stop on the New Jersey Turnpike some years ago had traveled to a dozen states to purchase bomb-making materials. By buying innocuous ingredients in different places, the expert said, this terrorist was trying to leave few clues to his true intent.

The expert said that while the presence of nitrates can in some instances indicate less-sophisticated explosives, the timing device or detonator, if it is recovered, could be the most valuable lead.

“The timing device may be much more illustrative of the sophistication involved,” he said.

Teams of bomb experts were flown into New York on a military transport plane to bolster the investigation.

Police disclosed Sunday that more than 50 phone calls claiming responsibility or offering information have been received since Friday’s blast.

Some of the phone calls were quite helpful, Fox told Cable News Network.

“We’ve already got quite a few good leads. Indeed, a surprising number of the calls turned out to be very helpful and very relevant,” he said.

New York Gov. Mario M. Cuomo said efforts were under way to find alternate downtown Manhattan sites for some businesses. He said the recession in New York had left many vacancies.

Officials hoped to reopen the twin towers before the week is over, but the prospect was uncertain because of extensive smoke damage and disabled safety and communications systems.

Millions of New Yorkers nervously greeted the prospect of a new workweek.

“If I were in a landmark building, a symbol of the nation, I might have second thoughts about going to work,” said Daniel Ezra Cohen, a Manhattan marketing executive. “I would be looking very carefully to see how many security people are present.”

“I am waiting to see which terrorist group is responsible. . . . There seems to be a line. Crazy people seem to be standing on line to take credit for this act of horror. We hope law enforcement can pick out the real culprit in this case.”

Leone said the World Trade Center was going to take costly security steps because of “this horrible thing that happened to us.”

He said he was concerned that one aftershock of the bombing could be the possibility that other buildings might have to take similar actions.

“One cost that will be hard to measure is if suddenly we have big costs of security imposed on large buildings all over the country,” he said.

Times staff writer Ronald J. Ostrow, in Washington, contributed to this story.

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