The e-mail was sent from somewhere in Pakistan at 7:14 a.m. Eastern Daylight Time on September 6, 2009. It was instantly logged by the massive data-gathering computers of the U.S. National Security Agency in Fort Meade, Maryland, and at GCHQ, the UK's signals intelligence agency.

The sender was someone known to U.S. and UK security services as "Ahmad," who'd been on the radar of British intelligence since a suspected al Qaeda cell had been uncovered in Manchester that year, according to senior U.S. counterterrorism officials.

But the recipient was previously unknown, with the address njbzaz@yahoo.com. Whoever it was lived in the Denver area. Alarm bells rang across the U.S. intelligence establishment. Who in Colorado was in touch with a man suspected as a handler for al Qaeda?

Within two hours, njbzaz replied, "Listen I need a amount of the one mixing of (flour and ghee oil) and I do not khow the amount."

Minutes later, he sent a follow-up: "Plez reply to what I asked u right away. the marriage is ready flour and oil."

U.S. authorities quickly established that the Denver-based e-mailer was Najibullah Zazi, a 24-year-old Afghan resident alien. He had moved to the Denver area from New York in January 2009 and taken a job as an airport shuttle van driver.

He appeared to be asking for clarification on the quantities of chemicals needed to make a bomb. Flour had frequently been part of the mixture in al Qaeda bombs in the West.

But U.S. intelligence agencies had no idea what Zazi was planning or whether he had co-conspirators. The FBI decided to track his movements and was soon trying to keep up with him.

Zazi left Denver in a rented Chevy Impala early September 8. The FBI was on his tail and asked a highway patrol officer to find a pretext to stop him -- and find out where he was going.

He told the patrol officer that he was driving to New York for a business meeting. He seemed nervous, the officer recalled, but was allowed to go after getting a written warning for speeding.

As Zazi drove cross-country at speeds exceeding 100 miles an hour, U.S. national security officials were in a state of high anxiety. "They were s**ting bricks," one source said. The anniversary of 9/11 was only days away, and President Barack Obama and other world leaders were soon set to travel to New York to address the U.N. General Assembly.

At 3:40 p.m. September 9, Zazi was approaching the George Washington Bridge into New York. The FBI instructed Port Authority police to set up a roadblock and include his car in a seemingly random stop and search operation. They brought in a canine to sniff around the vehicle but did not open the trunk. The dog detected nothing, and Zazi was allowed to continue into the city.

But in the trunk, hidden among clothes in a suitcase, was a jar in a plastic bag containing a powerful detonating explosive.

The path to terror

Three years earlier, Zazi had been working as a coffee vendor near Wall Street. Customers described him as cheerful. But he was gradually beginning to perceive the war in Afghanistan as hostile to Islam. And by 2008, he wanted to play a part in what he saw as the inevitable victory of the Taliban.

He was not alone. In recent court testimony, Zazi said that he and two others -- Adis Medunjanin and Zarein Ahmedzay -- swore an oath to join the Taliban outside a mosque in Queens. Ahmedzay said a Taliban group was operating near his family home in Afghanistan. Zazi would finance their trip, and buy computer equipment to give to the mujahedeen, by running up $50,000 in credit card loans.

In late August 2008, the trio traveled from New York to Pakistan. Days later, Medunjanin and Ahmedzay tried to cross the Afghan border but were turned back by Pakistani police. The trio then began searching for contacts in the city of Peshawar who could connect them to a jihadist group. An imam introduced them to a Pakistani man described by Zazi as in his mid-20s. He was the Ahmad of the e-mails.

Ahmad drove the three young men up into Pakistan's tribal territories but told them the group they would meet might want to send them back to America.

"We took it as a joke," Zazi said later, explaining that they were determined to fight in Afghanistan.

In a guest house near the town of Miram Shah, the trio were introduced to two senior members of al Qaeda's external operations unit: Rashid Rauf, a British operative who orchestrated the London bombings in 2005 and the 2006 transatlantic airline plot, and Saleh al Somali, a senior East African operative who headed up the unit.

Both wanted the men to return to the United States to plan an attack there. The trio explained that their intention was to fight in Afghanistan and that they did not want to return to the United States.

"Keep it in your mind. Take time to think about it," al Somali replied.

At an al Qaeda camp in South Waziristan, the group met American Adnan Shukrijumah, who had joined al Qaeda in Afghanistan in 2000. He also tried to persuade them to return to the United States, saying they could avenge the U.S. drone attacks and pressure the United States to leave Afghanistan. He told them that other al Qaeda operatives lacked their travel documents or comfort operating in the United States.

Eventually, after Shukrijumah became increasingly insistent, according to Ahmedzay, they agreed -- and discussion began about possible targets: the New York Stock Exchange, Times Square, Penn Station, Grand Central Station, movie theaters and the New York subway, because it was "the heart of everything in New York City," Zazi said later.

Shukrijumah suggested striking a Wal-Mart because it would damage the U.S. economy and told them that if they could not carry out a large attack, they should launch something smaller.