CHICAGO — Americans now have learned what the intelligence community knew for years: The most deadly terrorists may be living next door, act like typical suburbanites and look like they stepped out of a Gap ad.

But what even experienced counterterrorism experts did not know was how sophisticated and well-heeled many have become.

"We've learned that they have more money, more organization, and more coordination than we had known before or suspected before," said Anthony Cordesman, a senior fellow in strategic assessment at the Center for Strategic & International Studies, a nonpartisan research group in Washington.

Over the years, terrorists have mastered ways to alter their appearance and behavior to blend into any background, particularly in a nation as diverse as the United States. The fact that many of the Muslim suspects identified by the FBI in connection with the Sept. 11 attacks were described as clean-shaven men who wore jeans, drank alcohol and even frequented strip clubs — all anathema to fundamentalist Islam — did not surprise those who study terrorism.

Indeed, earlier this year, during the federal trial of those accused in the 1998 bombings of U.S. embassies in Kenya and Tanzania, a former close aide to exiled Saudi dissident Osama bin Laden testified about how even the most observant Muslims among his followers were taught to conceal their fundamentalist roots.

The former aide, Jamal Ahmed Al-Fadl, said that before crossing borders, members of bin Laden's terrorism group al Qaeda routinely were instructed to shave their beards, dress in Western clothes, smoke cigarettes, wear cologne and even carry pornographic magazines — all of which are forbidden by their religious beliefs.

Honing their abilities

Today's terrorists also are honing their ability to gather intelligence, employ technology and spot security gaps, experts say.

What clearly differentiates them from their predecessors of only a few decades ago is their ability to learn from their mistakes, to take advantage of new technologies and to come up with new tactics, anti-terrorism experts say.

For example, on Dec. 11, 1994, a bomb planted under seat 26-K of a Philippines Airlines jet bound for Tokyo exploded. One Japanese tourist was killed and 10 other passengers were injured, but the airplane made a safe landing.

The bombing was, in effect, a technical test by terrorists. It was one of many such tests, as Filipino police learned later when they caught the perpetrators, associates of bin Laden, in Manila.

The problem is that governments and security agencies sometimes concentrate on terrorists' old patterns or limit their focus to what the terrorists may be considering, experts say.

"It's an inclination to forget that terrorists are innovators," said Philip Wilcox, former U.S. ambassador-at-large for counterterrorism.

When Filipino police in January 1995 discovered the hideout of the terrorists who had planted the bomb on the Tokyo-bound plane, they also came across a laptop computer that detailed plans to put bombs aboard 11 U.S. planes flying over the Pacific.

The computer belonged to Ramzi Ahmed Yousef, who later was convicted by a New York jury of masterminding the 1993 bombing of the World Trade Center that took six lives and injured more than 1,000 people.

As part of his preparation to blow up U.S. planes, Yousef tested his planned technique on the Philippines Airlines jet. He had smuggled some explosive liquid aboard the airliner, assembled a crude bomb in a restroom and left it under a seat before leaving at an interim stop in the Philippines.

He wanted to see how the bomb worked because he planned to have terrorists do the same thing aboard U.S. passenger jets, prosecutors said.

In the U.S. attacks, the perpetrators used the fuel-laden planes as past terrorists used car bombs — as detonation devices. However, there was one big difference: the planes became flying, guided missiles armed with jet-fuel warheads.

"There was a clear failure of imagination by the U.S. to impose tough civil-aviation regulations years ago," said Wilcox. "In the absence of aerial hijackings, authorities said, 'We've done enough.' "

The terrorists also obtained valuable information from the first attempt against the World Trade Center.

After Yousef's arrest in 1995, he told a U.S. Secret Service agent that the goal of the 1993 attack was to topple the World Trade Center's twin towers with explosives left in vehicles parked under the buildings.

Trial and error

As a result of the failure of that attempt, the terrorists learned that the towers were sturdier than they had anticipated. Sept. 11, the perpetrators flew the two jets into the steel midsections of the 110-story towers. That, experts say, showed their intention to maximize the planes' destructive force.

Besides employing the element of surprise, the hijackers also chose long-haul flights with lighter passenger loads for them to manage. Staging the attacks almost simultaneously is a feat of considerable coordination, said Yoram Schweitzer, an expert with the International Policy Institute for Counter-Terrorism in Tel Aviv.

Terrorist groups not only collect and update information in the same fashion as international security agencies, but they also swap the information, said Ian Lesser, a senior analyst at Rand Corp. in suburban Washington.

Experts also said bin Laden is a strong believer in relying on the latest technology, such as satellite telephones, to speed communication among far-flung associates and to gather information.

The attacks on the Pentagon and the World Trade Center illustrated another increasing trend among terrorists: the apparent involvement of better-educated and better-trained people, such as pilots, who were willing to sacrifice their lives. Traditionally, terrorists have been desperate loners driven by an immediate thirst for violence. Such people still make up the ranks of terrorist groups, experts said. But they have been joined by a new cadre.

The Sept. 11 attack also underscored a new perspective from which Americans may assess terrorism's threat, Cordesman said.

In the past, America has drawn its sense of security from the threat of terrorism less from its own military might than "in the limits of the capacities of the terrorists," he said, adding, "obviously, those limits have changed."