The 2007 raid at Islamabad's Lal Masjid, where Faisal Shahzad often prayed when visiting his home, was the "triggering event" that drove the Pakistani-American to terrorism, culminating into the failed Times Square bombing plot.
The Red Mosque, believed to have become a hub of extremist activity, was stormed by the forces of former President Pervez Musharraf in 2007 after days of siege. Nearly a 100 people died in the raid.
Lal Masjid was the mosque where Shahzad had offered prayers on his frequent trips to Pakistan.
"That was the triggering event," a person familiar with the case told the New York Times, which pieced together bits of Shahzad's journey on the terror path from interviews with people who are currently involved in his interrogation.
Musharraf had defended the raid, saying it was prompted by intransigent militants who had "challenged the writ of the government". He said the mosque was being misused by extremist elements.
Three years after the Red Mosque raid, 30-year-old Shahzad, son of a retired Air Vice Marshal, drove an explosive-laden Nissan Pathfinder to Times Square on May 1.
This week, Shahzad pleaded guilty to 10 terror and weapons charges.
During the court hearing, he described himself as a "Muslim soldier" and warned that the US would face more attacks if they continued operations in "Muslim lands".
"One has to understand where I'm coming from... I consider myself... a Muslim soldier," he said, expressing anguish at the continued presence of US in Muslim countries like Afghanistan and Pakistan as well as the drone attacks on the Af-Pak border.
Shahzad, who will be sentenced on October 5, was apprehended at John F Kennedy airport trying to escape to Dubai and has been cooperating with the federal authorities by providing them with information.