JOIN USDuring a visit to Pakistan in 2008, Shahzad gave perhaps the clearest indication yet that he was heading down a militant path, the New York Times reported.
"He (Shahzad) asked his father (Bahar-ul-Haq) for permission to fight in Afghanistan, friends of the father and the relative recalled. Mr Haq denied the request and appealed to the friends for help in managing his son," the paper quoted them as saying.
It also said that Shahzad began to have clashes with his father on his visits to home.
Haq, who had long been wary of political Islam, found his son's evolution troubling, friends recalled in interviews.
"The scrutiny went both ways. Shahzad glared when his father once asked him to fetch water to mix with his whiskey, a family friend recalled. He wanted to change his father," said a classmate.
But the roots of Shahzad's inclination to militancy appear to have sprouted long before, according to interviews with relatives, friends, classmates, neighbours, colleagues and government officials, as well as e-mail messages written by the 30-year-old naturalised American obtained by the Times.
Quoting an e-mail sent by Shahzad on February 25, 2006, to a group of friends, the paper says the trials of his fellow Muslims, the wars in Iraq and Afghanistan, the plight of Palestinians, and incidents like the publication in Denmark of cartoons lampooning the Prophet Muhammad, weighed on him.
"Mr. Shahzad was wrestling with how to respond. He understood the notion that Islam forbids the killing of innocents, he wrote. But to those who insist only on 'peaceful protest,' he posed a question: 'Can you tell me a way to save the oppressed? And a way to fight back when rockets are fired at us and Muslim blood flows?
As Shahzad became more religious, starting around 2006, he was also turning away from the Pakistan of his youth, his friends recalled, distancing himself from the liberal, elite world of his father.
In April 2009, the same month Shahzad got his United States citizenship, he sent an e-mail message to friends that foreshadowed his militant destiny.
He criticised the views of a moderate Pakistani politician, writing, "I bet when it comes to defending the lands, his opinion would be we should do dialogue".
One of the recipients responded by asking Shahzad which sheikhs he followed, to which Shahzad replied, "My sheikhs are in the field".
A few months later, he abruptly quit his job and left for Pakistan, where, officials say, he was later trained in bomb-making by the Pakistani Taliban.
But precisely what combination of influences — political, religious and personal — drove Shahzad to violence remain a mystery, even to those close to him.
"We all know these things, what the geopolitical problems are," said Shahzad's father-in-law, M A Mian.
"But to go to this extreme, this is unbelievable," he said, adding: "He has lovely children. Two really lovely children. As a father I would not be able to afford to lose my children".