The informant was not identified in court papers unsealed on Wednesday in Manhattan. But according to a person briefed on the case, the informant is Shahed Hussain, the central prosecution witness in a 2004 federal sting focusing on a pizzeria owner and an imam at an Albany mosque.

Lawyers for those men argued that Mr. Hussain, who had posed as a wealthy Muslim radical, had entrapped their clients in an ultimately fictional plot to kill a Pakistani diplomat with a missile. But a federal jury convicted the two men, and they were sentenced to 15 years in prison.

“Any defense attorney worth his salt is going to argue entrapment,” Raymond W. Kelly, the New York police commissioner, said Friday when asked about the use of an informant in the Newburgh case. “The argument will be made in court. But in essence, the law says you have to be otherwise not disposed to do the crime to successfully use the defense of entrapment.”

The government’s court filings present the informant as someone who merely assisted the violent intentions of the four men. Federal authorities have asserted that one of the defendants, James Cromitie, was angry about the war in Afghanistan and was determined to strike at America, and later at Jews. The informant, who told the men he had connections to a Pakistani terror group, then provided the men with what they believed to be sophisticated explosives and a missile.

Asked whether he thought the four men were a serious security risk before they were approached by the informant, Joseph M. Demarest Jr., who heads the F.B.I.’s New York office, said: “It was their plot and their plan that they pushed forward. We merely facilitated. They asked for the explosives. They asked for the Stingers, or rockets, I think, is the way they described it. They did leave the packages of what they believed to be real explosives, the bags, in front of two temples in the Bronx.”

Vincent L. Briccetti, who represents Mr. Cromitie, said he was aware of Mr. Hussain’s role in the Albany case, which was reported on Friday in The New York Post.

“His history is of interest to us,” Mr. Briccetti said.

Court records from the Albany case show that Mr. Hussain came to the United States from Pakistan in 1993 or 1994. He appears to have held a variety of jobs, and come to own a number of businesses and properties. But in 2002, he was charged with a scheme involving taking money to illegally help people in the Albany area get driver’s licenses.