After the failed attempt to bomb Times Square, New York police are dispatching more officers to be seen on the streets, around landmarks and on subways.

But there’s one tactic they hope won’t go noticed at all: getting inside the bands of terrorists-in-the-making.

That’s why a young Bangladeshi immigrant working undercover found himself among a dozen men at an Islamic bookstore in Brooklyn one day in 2004 to watch videos of U.S. soldiers being slain.

“That made these guys pumped up and happy,” the officer said. “It’s like a party at a club. They were hitting the walls with excitement. One guy even broke a chair.”

Among the revelers: Shahawar Matin Siraj, who would be sentenced in January 2007 to 30 years in prison for an August 2004 plot to blow up Herald Square. “He loved talking about doing jihad,” said the officer.

In an interview with The Wall Street Journal, the undercover officer described four years embedded with Brooklyn radicals, a stint which began a few months after the Sept. 11 terror attacks and ended with his testimony at Mr. Siraj’s trial in mid-2006.

Police and the officer declined to make his identity public. In court records in the trial of Mr. Siraj, he was identified by his undercover name, Kamil Pasha.

David Cohen, deputy commissioner for intelligence of the New York Police Department, said such undercover operations have become the city’s main defense amid the escalation of threats and plots since the attack on the World Trade Center nearly a decade ago.

The 30-year-old officer spent his childhood in Brooklyn and Queens, where he went to high school. He joined the force after graduating from the John Jay College of Criminal Justice in 2001. He said his undercover work has remained a secret to his friends, siblings and parents. During the posting, he told his parents he was working for a private security firm, and they now know he works for the police department.

He received individualized training so few would know he was a police officer; there would be no buddies from the academy to recognize him on the street. He said undercover investigators must walk a delicate line by playing the role of a potential terrorist and friend while refraining from pushing a plot forward.

The officer said only a few other members of the department knew of the life he developed in Brooklyn, as he rented an apartment, bought furniture, joined a local gym and slowly sought to become part of the community.

Mustafah Abdulaziz for The Wall Street Journal Deputy Commissioner of Intelligence David Cohen

He attempted to maintain as much of his everyday personality as possible; he didn’t change his habit of attending a mosque with some regularity, and he sought to make friends among the community.

The officer said he fit the profile of the young men he sought to meet: middle-class, first- or second-generation Americans in their late teens or early 20s. He said he watched the radicalization process of dozens.

At times, it was so rapid that a year or two could separate clubbing in Miami from prayer five times a day.

The officer described Mr. Siraj’s path. It unfolded in Brooklyn mosques, on local basketball courts and at an Islamic book store in Brooklyn that served as a gathering spot for radicals. The video, for example, that the officer said he watched with Mr. Siraj showed the “top 10” killings of U.S. soldiers in Iraq.

The groups he penetrated spoke frequently of jihad, or holy war, and enlisted him to train with them. By the time an attack on Herald Square was being plotted, the officer had decided to focus his time on another group in Borough Park that had converted to Islam while in prison.

Once with that group, he trained for jihad by going paintballing, climbing mountains late at night, shooting assault rifles at firing ranges. During one of these trips to a firing range, he says he felt the barrel of a 9mm handgun pressed to the back of his head.

The officer said he was able to talk the youth down, though to this day he said he still doesn’t know if he was being tested. Back at police headquarters, Mr. Cohen said officials mulled for days over whether to pull him from the assignment.

Eventually, the officer surfaced to testify in the case against Mr. Siraj, who claimed he had been entrapped by a government informant—not the officer himself—to bomb Herald Square before the 2004 Republican National Convention. Explosives were never obtained for the attack.

The undercover program is both secretive and controversial. Local Muslim groups have criticized the infiltration of the Muslim community by investigators from the Intelligence Division as a form religious profiling.

The police deny that, saying they follow threats wherever they may lead.

Mr. Cohen declined to say how many undercover officers work for the department or in counterterrorism.

Police spokesman Paul Browne said there are about 1,100 people assigned to counterterrorism throughout the department, with more than 300 of those in the Intelligence Division. Despite the reduction of the overall uniformed force—from 41,000 to 35,000 in the last eight years—Police Commissioner Ray Kelly has not scaled back the Intelligence Division.

Write to Joel Stonington at Joel.Stonington@wsj.com