The case is the latest in a series in New York and around the country since Sept. 11, 2001, and sounded familiar in some ways. The investigation, for instance, began with the work of a confidential informant, who portrayed himself as an agent of a Pakistani terror organization, and who became a critical member of the men’s plot.

The full nature and extent of the informant’s role in facilitating the plot is unknown. In other cases, defense lawyers have sought to portray these informants as engaging in entrapment, suggesting they had, in effect, provoked and fueled the actions of their clients.

Image Police Commissioner Raymond W. Kelly, right, and Mayor Michael R. Bloomberg at the Riverdale Jewish Center on Thursday. Credit... Rob Bennett for The New York Times

But where past terror prosecutions have been based mostly on conversations about a planned or imagined attack, this one went further, the authorities alleged: the men went through critical acts in what they believed to be a deadly assault.

As for the defendants  James Cromitie, 44; David Williams, 28; Onta Williams, 32, and no apparent relation to David; and Laguerre Payen, 27  most of the details that emerged on Thursday stemmed from their criminal pasts.

David Williams, who lately had grown a beard and taken to reading the Koran on slow nights at a steakhouse job, was described as particularly violent by prosecutors on Thursday. When the plan to buy guns from a woman in Newburgh fell through, it was David Williams who quickly improvised, arranging to buy a gun from a man he described as a “supreme Blood gang leader” in Brooklyn, Mr. Snyder said. After buying the gun in the company of the informant, David Williams said he would have shot the gang leader if he were alone with him, and kept his $700.

Mr. Payen, described as a nervous, quiet sort who took medication for schizophrenia or a bi-polar disorder, was unemployed and living in squalor in Newburgh. His last arrest, in 2002, was for assault, after he drove around the Rockland County village of Monsey, firing a BB gun out of the window  striking two teens  and snatching two purses. A friend who visited Mr. Payen’s apartment on Thursday said it contained bottles of urine, and raw chicken on the stovetop.