Afghan native Najibullah Zazi confessed to plotting to use weapons of mass destruction in a suicide bomb attack on the New York subway.

They are middle-class, some (by their home country's standards) even well-off. They are often college educated. They are settled in the United States or elsewhere in the West, far from the chaos or sectarian strife of their homelands; they are supposedly "assimilated." But somehow they cast off a life of comfort and drift toward extreme views before embracing political violence inspired by a sense of grievance or alienation.

It is a pattern seen time and again as terrorist plots have been uncovered in the United States. Afghan native Najibullah Zazi; Pakistani-American David Headley; Bryant Neal Vinas, the U.S.-born son of Latino immigrants; and Nigerian student Umar Farouk Abdulmutallab, charged with trying to bring down an airliner over Detroit, Michigan, on December 25.

Zazi, who confessed to plotting to use weapons of mass destruction in a suicide bomb attack on the New York subway, was not well-off. But his family was well-established in the United States. His uncle in Denver, Colorado (with whom he lived for part of 2009), owns a spacious house in a pleasant suburb. Zazi attended High School in Flushing, New York, and although religious showed no signs of Islamist militancy as a student. He played billiards and basketball and later ran a coffee-cart business in Wall Street. His patrons described him as likeable, with a ready smile.

Vinas also had a comfortable middle-class upbringing in Long Island and was a baseball fanatic. Neighbors and friends describe him as a courteous, respectful student. Rita Desroches, a neighbor whose son was a good friend of Vinas', describes him as a "very sweet little guy. He could come here any time any minute. Just walk in. He was always welcome."

Abdulmutallab, the young Nigerian who is accused of trying to blow up a U.S. airliner, had a privileged upbringing. He attended one of West Africa's best schools: the British School in Lome. His father is a prominent banker in Nigeria; the family had an expensive apartment in London, England, where Abdulmutallab studied mechanical engineering. He traveled widely - to the United States and the Persian Gulf.  He has pleaded not guilty to charges including attempted use of a weapon of mass destruction.

Headley was born in Pakistan to a distinguished Pakistani diplomat and his American socialite wife. He had a privileged upbringing, attending an elite Pakistani military school and moving easily between the worlds of East and West. But his parents separated when he was a teenager, and he came to live in the U.S. with his mother. He dabbled in the drug trade, working as a courier of heroin from Pakistan to the U.S. until being apprehended in 1998. But even as he ran afoul of the law, there was no sign of Islamist militancy.

The investigation into Times Square suspect Faisal Shahzad's background reveals a similar story. His father is a retired senior Air Force officer in Pakistan, and the family home is in a comfortable suburb of Peshawar. For a while they lived in a two-story villa in Karachi when Shahzad's father was a senior official in the country's aviation authority. Shahzad was well-educated and attended colleges in Pakistan and Bridgeport University in Connecticut. His wife received a degree from the University of Colorado at Boulder and published several books. They purchased a new house in Shelton, Connecticut, and he commuted to work in New York's financial district. He was not a high earner or high performer, according to former employers, but he had a respectable, steady job and two children.

Even Maj. Nidal Hasan, the Fort Hood shooting suspect, would hardly be the "stereotypical" jihadist. He was a career soldier, born in Virginia, and a qualified (though apparently not very accomplished) psychiatrist.

Counterterrorism officials and experts on radicalization say that although there are differences in the backgrounds of these men, there are also striking similarities. In some way, they are affected by a change in their personal lives, grievances fed by a sense of injustice, a search for identity or belonging, a sense of alienation from their social environment. Often they are gullible and impressionable.

Vinas - the subject of a series this week on CNN - was traumatized by his parents' separation. CNN Terrorism Contributor Paul Cruickshank, who has spoken with his mother and sister at length, says: "There were tears and temper tantrums. He started quarreling with his sister, being disrespectful to his mother. He refused to accept his parents' separation."

Vinas, according to family and friends, was continually searching for a sense of identity and purpose in his life. After spending a few weeks in the U.S. Army and realizing that a military career was not for him, Vinas was searching for meaning in his life. He found it when he met the brother of a friend who was a Muslim. Vinas asked questions about Islam, and the brother gave him a Quran. Attending a mosque and embracing Islam with the passion of a convert gave him a sense of identity, and he began to believe the grievances of radical Muslims he encountered about U.S. policy overseas and especially in Afghanistan. He ended up booking himself on a flight to Lahore, Pakistan - his aim to join the jihad against U.S. forces over the border.

Carvin Desroches, one of Vinas' best friends growing up, says Vinas was the last of his friends he would have expected would do such a thing. Vinas' mother and sister say that if this happened to their son, they fear it will happen to another American family.

New York Police Department Intelligence Analysis chief Mitch Silber says Vinas "is almost a poster child for the process, the unremarkable nature of the people who might go through this process and frankly the potential to link up to al Qaeda and the danger that presents."

There appears to be no single moment when Zazi was radicalized and no obvious influence on him. But as with Shahzad, he ran into financial difficulties. One customer at his coffee cart told The New York Times that Zazi rebuked her one day. "He told me I could not be happy. He said: 'You people cannot be happy with your money.' "

Zazi filed for bankruptcy in March 2009 with credit card debts of more than $50,000. A few months later, he and two former school-friends left for Pakistan, where Zazi has admitted he received explosives training.

Headley's motivations remain obscure. He appears to have linked up with Pakistani militants while involved with drug trafficking. His dual nationality and ability to move in elite circles were a potent combination with an appetite for adventure and risk. But there were few signs of Islamic militancy, even when he confessed his role in planning the Mumbai attacks by Lashkar-e-Taiba and the plot to bomb the Danish newspaper that had published cartoons depicting the Prophet Mohammed.

Both Hasan and Abdulmutallab were conflicted over their relationships with women. Hasan's failure to find a wife who would wear a veil haunted him, but at the same time he is said to have visited a strip club near Fort Hood, Texas. Abdulmutallab agonized in his blog entries over finding a "modest" wife. And Anwar al-Awlaki, the Yemeni-American preacher whose views on jihad have influenced so many "home-grown" jihadists, was twice arrested in California for soliciting prostitutes.

Both Hasan and Abdulmutallab appear to have been alienated from their "decadent" surroundings and regarded Western society as morally "flawed."

But the alarming feature for intelligence officials in all these cases is that they defy the likely profile of a terrorist. There is no obvious red flag. These are people who appear to be "ordinary" members of society. They are U.S. citizens or resident aliens (who can therefore move in and out of the country with ease.) They do not belong to readily identifiable radical groups and have not spent their childhoods in radical madrassas.

Many of the home-grown jihadists became loners; family members have no hunch of what they are doing. (This has also been the case with young Somali-Americans who have suddenly disappeared from homes in Seattle, Washington, or Minneapolis, Minnesota, to fight a holy war in the Horn of Africa.). They are often influenced by radicalizers who stress that their family is less important than their duty to Allah. Hasan communicated with al-Awlaki; Abdulmutallab may have done so, too. (It is remarkable how many of the conspirators in the U.S. and UK have been influenced by al-Awlaki's religious justifications for jihad.)

These individuals are the opposite of the hardened fighters of groups like Hamas and Hezbollah and the Pakistani Taliban, who have grown up in poverty amid a collective sense of oppression. For example, Hezbollah was born among downtrodden Shiites in the slums of southern Beirut, Lebanon. The Pakistani Taliban, now allegedly linked to Shahzad's attempt, overwhelmingly comprises poorly educated and often illiterate young men from rural parts of northern Pakistan.

If the assertion by senior U.S. officials is correct, and Shahzad did link up with the Pakistani Taliban, the young madrassa-educated militants were teaching the bilingual MBA graduate how to bring terror to New York.