hey were adults with education and skill, not hopeless young zealots. At least one left behind a wife and young children. They mingled in secular society, even drinking forbidden alcohol, hardly typical of Islamic militants.

Some of the men who are suspected of hijacking four airplanes in the world's worst terrorist attack do not fit the profile of the suicide bombers who have plagued the Middle East, Sri Lanka and Chechnya over the past two decades. Most of those self- proclaimed martyrs had little to lose, and were indoctrinated for short, intense periods between recruitment and their deadly missions. In contrast, those suspected of perpetrating Tuesday's destruction had, in some cases, spent years studying and training in the United States, collecting valuable commercial skills and facing many opportunities to change their minds.

"What we see here is a totally new pattern," said Ehud Sprinzak, a terrorism expert and the dean of the Lauder School, a public policy institute in Herziliyah, Israel. "We have published a book on suicide bombing, but now we'll have to rewrite the book. This is staggering new evidence."

This week's events differed not just in scale, but also in the fact that the hijackers died in groups. Preliminary evidence about the suspected terrorists also suggests that they were not reckless young men facing dire economic conditions and dim prospects but men as old as 41 enjoying middle-class lives. Just last week, even those numbed to suicide bombings in Israel were shocked by the latest incident there because the perpetrator, an Israeli Arab, was 48 and a father.

Experts called it too early to say what the demographic differences might mean about the shifting dynamics of international terrorism. Perhaps, they said, loyalty to Osama bin Laden is even more powerful than the religious and nationalist fanaticism that has been behind other suicide attacks. Perhaps the size of the target attracted more sophisticated candidates. Or perhaps the hatred of the United States and Western culture is seeping into a broader spectrum of the world's disaffected populations.

"People who have a lot of other reasons to live for are deciding that this is such an important cause that they're willing to die anyway," said Andrea Talentino, a political science professor at Tulane University who specializes in security studies. "That, obviously, is very frightening."

The concept of the suicide bomber dates to the 11th century, when the Assassins adopted it as a strategy to spread Islam through northern Persia. It appeared again among Muslims from India to the Philippines in the 1700's. During World War II, Japanese fighter pilots were recruited for suicide, or kamikaze, missions.

Today, suicide bombings are a prime tool of terrorism. Researchers documented 286 incidents from 1983 to 2000 in Lebanon, Israel and Turkey, but bombings were also a part of the civil war in Sri Lanka, where the Liberation Tigers of Tamil Eelam formed elite army units for such missions and used them to assassinate two heads of state. Among recent suicide attacks were the 1998 bombings of the American Embassies in Kenya and Tanzania.

The prototype for Muslim suicide bombers is young, single, caught up in religious fervor and, often, desperate. They are usually promised financial security for their parents and told that they will be greeted by 70 black-eyed virgins in heaven. Though suicide is prohibited by Islamic law, some leaders have said there is an exception for soldiers in what they see as a holy war.

"We have nothing with which to repel killing and thuggery against us except the weapon of martyrdom," Dr. Ramadan Shalah, secretary general of the Palestinian Islamic Jihad, was quoted as saying in an article by Mr. Sprinzak last year in Foreign Policy magazine. "It is easy and costs us only our lives."

In the article, Mr. Sprinzak cited the tactical advantages of suicide terrorism: no escape routes or rescue operation are required; there is no risk of captured perpetrators divulging information; and the public feels extraordinarily helpless.

"One of the virtues of the suicide bomber is that it's very simple technically ?no sophisticated detonators, no time delays ?it's much simpler to bring off and thus you're much more likely to get through," said Martha Crenshaw, a professor of government at Wesleyan University who specializes in the issue. "This operation, the hijackings, was very complicated. You certainly needed more than one person to pull it off. You had to have a small group of people who worked together, who knew each other and trusted each other."

Small group dynamics, Ms. Crenshaw said, may propel the mission beyond any individual's commitment. "What keeps them fighting is what keeps soldiers in a platoon fighting," she said. "They don't want to let their buddies down."

Ariel Merari, a political psychologist at Tel Aviv University who is writing a book on suicide bombers in Lebanon and Israel, said the average age of the 74 he studied was 22. Documents show that one of this week's suspected hijackers was 41, another 33; two were 28, two 26 and three 25 (ages were not available for all 19 suspects).

Mr. Merari's study of previous bombers showed that virtually none were married or engaged. But investigations of the suspected hijackers reveal suggest that Abdulaziz al- Omari, one of those aboard the plane that hit the north tower of the World Trade Center, lived with his wife and four children in a stucco house near his Florida flight training school. And contrary to the image of the fundamentalist Muslim, Mohammed Atta, who was aboard the same plane, was seen drinking and playing video games at a Florida sports bar last week.

Ms. Crenshaw said that seemingly secular activity could have been part of a ruse, noting that a training manual cited in the embassy bombing trials instructed suicide bombers: "When you're in the outer world, you have to act like them, dress like them, behave like them."

Stuart Grassian, a psychiatrist at Harvard Medical School who examined some of the suspects in the embassy bombings, said evidence of older, better educated and more stable suicide soldiers might indicate that individuals' rage had resonated to become endemic to a culture.

"The kind of horrifying prospect is that Osama bin Laden and what he represents has sort of crystallized a moment in history that has an evil and a horror to it that's sort of akin to what Hitler was able to crystallize around him," Dr. Grassian said.