Investigators were questioning the alleged Times Square bomber yesterday over possible links with other terrorist groups. But they also were seeking an answer to perhaps the biggest riddle of all: why does a young man with a wife, two young children, a newly acquired US citizenship and a good job throw away everything to stage a potentially lethal attack against his country of adoption?
Faisal Shahzad is the latest example of a breed which has vastly complicated the job of those investigators: the home-grown extremist who may or – as in the case of Colleen Renee LaRose, the Philadelphia woman known as "Jihad Jane" – may not be of Islamic origin, but is apparently well integrated into American society.
Mr Shahzad is 30 years old and (like several of the original 9/11 hijackers) from a good family. The son of a former air vice marshal in the Pakistani Air Force, he arrived in the US more than a decade ago as a student. At college he did not stand out; when he set up home with his family in suburban Connecticut, he attracted little attention. Neighbours described him as "normal," if somewhat private and uncommunicative: "a little weird", one said. The estate agent who sold him his house in 2004 remembers Shahzad as being fiercely critical of the Iraq war – but by then millions of Americans felt the same.
Otherwise he gave no hint of radical leanings; he was never even seen at the local mosque. The only "Islamic thing" about him, another neighbour said, was his short beard. Otherwise, Mr Shahzad's taste in dress, in public at least, tended towards the all-American slacks and blazer.
His upbringing in Pakistan, meanwhile, hardly seems to have pointed to a radical future. He was brought up near Mohib Banda, a village outside Peshawar. Though he left was he was 18, locals in the farming community of just 5,000 people have expressed disbelief the young man could have been behind the Times Square plot.
Family friend Kifayatullah Khan, a lawyer, said locals believed the young man had been framed. "They are highly respected in the village," he told Dawn newspaper. "The entire family is highly educated and enlightened. The villagers don't believe that Shahzad could act in such a manner."
In retrospect 2009 appears to have been the pivotal year. A graduate from the University of Bridgeport, Mr Shahzad by then had been working for three years as a financial analyst at a private equity firm.
In April 2009 he took the oath of allegiance as an American citizen. But within two months he had given up his job and stopped mortgage payments on his house, and left for Dubai, and then Pakistan. His wife Huma Anif Mian and children left as well, and the house fell into foreclosure in September.
By that time however, US investigators say he was being trained in bomb-making by extremists in northwestern Pakistan, though that is disputed by Pakistani officials. The people of Mohib Banda say he returned for a five-month stay with his parents, and attended a local wedding. Pakistan's Interior Minister, Rehman Malik, has said Mr Shahzad had made eight to 10 visits in the past seven years. He insisted that whatever Mr Shahzad's confession, there was no evidence of any links to militants.
It was equally unclear whether the failed operation was in fact sponsored by the Pakistan Taliban – despite claims to that effect immediately after the Nissan Pathfinder, loaded with propane, fireworks and fertiliser, was discovered, primed to explode, in the tourist heart of New York.
In February 2010 Mr Shahzad flew back to the US on a one-way ticket, bought for cash. He told immigration authorities he had been visiting his family. He moved into a sparsely furnished two-room apartment in Bridgeport, and seems to have devoted his energies to his plot. Two weeks ago, he paid $1,300 for the car, again cash. He loaded the vehicle and on Saturday dumped it in Times Square.
As a terrorist, Mr Shahzad was not the most accomplished. He forgot to remove the vehicle identification number from the Pathfinder's engine, enabling police to quickly trace its history. They also had little difficulty in piecing together the calls, within the US and to Pakistan, that he made with a disposable mobile phone.
By the time he tried to flee to Dubai on Monday evening on an Emirates plane, he knew the police were on his heels. "I was expecting you," he told agents as they entered the plane to remove him, according to US media yesterday. "Are you NYPD [New York Police Department] or FBI?"
Now the authorities must find out just how many other would-be terrorists of similar background are at large in the US. Like Mr Shahzad, Muslim-Americans tend to be well educated, with above-average incomes. Any threat, experts say, is posed by only a very, very few. But that threat exists.