Sept. 20, 2001 -- A new breed of terrorist has been introduced to America. They are calculating, as well-trained as any soldier, and know how to blend in without attracting attention to their murderous intentions.
"The profile of the World Trade Center and Pentagon terrorists differs from earlier profiles," says Dr. Mark Levy, a forensic psychiatrist at the University of California San Francisco. "These men were older, more educated, and some had wives and families."
A prime example is the hijacker authorities have identified as Mohamed Atta, the 33-year-old family man who is said to have trained as a pilot for years before flying a passenger jet into a World Trade Center skyscraper filled with office workers.
In the past "most suicide bombers were single, disenfranchised, depressed and infused with a religious fervor," says Gregg McCrary, a retired FBI special agent.
He points to a recent study in Israel of 74 suicide bombers, among whom the average age was 22.
By contrast, the estimated 19 men who hijacked and crashed four passenger jets last week were well-trained, focused, and often had years of experience and a particular expertise. They would have seemed to have had much to live for.
Not Insane; Committed
While their acts were horrendous, experts agree the terrorists were not "crazy" or insane. Insanity would imply they did not understand the wrongfulness of their acts or the death and destruction that would result.
"It is unlikely that any of the terrorists suffered from a serious mental illness," says Dr. Park Dietz, the head psychiatric consultant for the FBI and the founder of the Threat Assessment Group Inc., a private forensic consulting firm in Newport Beach, Calif.
In fact, quite the opposite may have been true because "in order to be chosen for such a mission, [the terrorists] would need to prove themselves trustworthy, reliable, and dedicated" to a cause, he says.
Commitment to that cause, experts say, can be borne of personal experience with terror or violence, or a feeling of being persecuted.
While it is not clear what motivated Atta, there are some clues about his behavior and actions.
Born in a rural town in Egypt, he appeared to have gone through some change while studying urban planning at Hamburg Technical University in Germany, where he enrolled in 1992.
"We observed that there was a changing between '95, and '98-'99 when he made his diploma," says Professor Dittmar Machule, Atta's thesis supervisor.
Machule says Atta abruptly shaved off his beard and seemed to be a changed man, as if he had a split personality.
"The only explanation I have now is that there was a cut in his brain. It was somewhat astounding," Machule says. "Of course, we all asked him what happened, and he explained it with trouble in his family."
Fifteen months ago, Atta came to the United States to study flight training. Patrons of a Hollywood, Fla., bar he frequented and a neighbor describe him as intense yet emotionless.
"He was not a nice guy, like, he wasn't friendly," says upstairs neighbor Carmen Padilla. "He was just quiet and had no emotion whatsoever, none. You can look at this person, you can feel nothing from him."
No Suicidal Symptoms
While the experts can't assess why last week's hijackers, who died in the attacks they carried out, did what they did, the psychological motivation could stem from "rage and a sense of self-righteousness," says Dr. Harold Bursztajn. He is co-director of the program in psychiatry and the law at Harvard Medical School, has profiled terrorists and often testifies as an expert witness in mass-murder trials.
These individuals do not display symptoms of those who typically contemplate suicide, Bursztajn says. They are not "depressed, hopeless, and helpless" because "their primary aim is not suicide" but the accomplishment of an awful mission, he says.
Dietz adds that the terrorists took part "in a suicide mission on behalf of a cause … with the expectation of contributing to the accomplishment of a greater good."
He points out that while "the terrorist on a suicide mission seeks revenge and publicity … their primary goal is political change."
It appears that suicide is a mere consequence of their mission, and the "terrorists see themselves as soldiers willing to sacrifice themselves for a higher purpose."
McCrary, who was in the FBI for 25 years, says that because of their belief system or ideology, "most Middle Eastern religious terrorists believe that they are going on to a greater eternal reward through their actions" and that there is some type of "personal gain to be achieved through their homicidal and suicidal behavior."
The ability to accept one's death "is easiest for those who expect a rewarding afterlife," says Dietz.
Blending Into American Society
The experts also say they are not surprised the terrorists could live undetected among those they wish to kill. Many of the accused allegedly lived and traveled in the United States for months or years.
The University of California's Levy explains that some people have the "capacity to divide their consciousness" into two separate and often conflicting identities. A terrorist living in America may be able to "isolate behaviors" that even they find morally reprehensible while they at the same time live in a community, have a family, and "blend in as a normal person," he says.
Unlike a serial killer, who feels some connection to the victims, "these terrorists are very impersonal; they see us as insects to be destroyed," says Bursztajn.
They are capable of killing a neighbor they know well, or a faceless victim in the World Trade Center, he says. They can live among us and still kill us "just as a husband might kill his spouse and children even though he lives with them for years" and shows the outside world no signs of violent intentions.
Like double agents trained by a government, "the most important trait of those who can lay low for a prolonged period is a strong affiliation with the group to whom they are loyal," says Dietz.
There may have been very few signs of the terrorists' intentions, perhaps only "saying goodbye to loved ones and setting their affairs in order," says Dietz.
McCrary adds that the terrorists would not "telegraph their intentions overtly" because of the fact that "these offenders appeared to be quite disciplined."
Burstajn says the only way to spot anyone affiliated with the hijackers is to "beware of those who celebrate the mass murderers and those who apologize for them," perhaps indicating some sense of guilt by association.
Professor Harvey Clifton of New York's Long Island University, an authority on terrorist groups, believes there are more like Atta at large, quietly blending in, impossible to detect.
"He's a faceless enemy," Clifton said. "There is no profile available to keep individuals like this off planes, out of bars, away from hotels or in flight schools."
ABCNEWS investigative correspondent Brian Ross contributed to this report.