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Everyone called the stranger with all the money “Maqsood.” He would sit in his Mercedes, waiting in the parking lot of the mosque in Newburgh, N.Y., until the Friday prayer was over. Then, according to members of the mosque, the Masjid al-Ikhlas, he approached the young men.

He asked Shakir Rashada, 34, if he wanted to come over for lunch. He offered Shafeeq Abdulwali, 39, a job, perhaps at his construction company. Jamil Muhammed, 38, said he was offered cellphones and computers.

The man, a Pakistani, occasionally approached the assistant imam of the mosque, proposing meetings, or overpaying for a sandwich he would buy at a mosque fund-raiser. In time, many of the mosque’s older members had made the man for a government informant, according to mosque leaders. They said that he seemed to focus most of his attention on younger black members and visitors.

“It’s easy to influence someone with the dollar,” said Mr. Muhammed, a longtime member of the mosque. “Especially these guys coming out of prison.”

The members of the mosque now believe that Maqsood was the government informant at the center of the case involving four men from Newburgh arrested and charged this week with having plotted to explode bombs at Jewish centers in New York City. The government has said that the four men, several of whom visited the mosque in Newburgh and all of whom spent time in prison, were eager to kill Jews, and prosecutors charged that they had actually gone so far as to plant what they believed to be bombs on the streets of New York, an act the F.B.I. captured on videotape.

The government case revolves significantly around the work of an informant who facilitated the men’s desire to mount a terrorist attack.

The role of informants has been a constant in the terror cases made by federal and local authorities since 9/11. And just as constant have been the attempts by lawyers for those charged to portray their clients as dupes, people who would not have committed to do harm without the provocation of the informants.

Those attempts have typically failed. Juries, evidently unmoved by claims about the conduct and influence of the informants, have routinely convicted those charged in the terror plots, like the five men charged with wanting to kill soldiers at Fort Dix in New Jersey, and the young Pakistani immigrant from Queens charged with conspiring to plant a bomb in Herald Square.

And, it turns out, an entrapment defense failed in a case involving the informant in this week’s bomb plot investigation.

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.

To avoid being deported, he agreed to assist the government — first taking part in a sting aimed at the driver’s license scheme, and later in a heroin trafficking case. In 2003, the F.B.I. enlisted him in a more ambitious case. They wanted him to help them learn more about the intentions of a man who they worried might be supportive of terror, Yassin Aref, and toward that end, began to focus on his friend Mohammad Mosharref Hossain.

Under the coaching of a federal agent, and often wearing a recording device, he met with the men, and presented himself as what he later at trial called “a wealthy radical.” Eventually, the government charged the two men with money laundering as part of a plot to acquire missiles, and perhaps use one to kill a Pakistani diplomat.

Mr. Hussain testified at length at the trial of the two men, and defense lawyers sought to portray him as a tool of an overly zealous government.

He said that he met with an F.B.I. agent before every encounter with the two men to go over his game plan.

“What Agent Coll used to tell me, I used to tell them exactly,” Mr. Hussain testified under cross-examination about his dealings with the F.B.I. agent and the two men.

“So you did exactly what Agent Coll told you?” he was asked by a defense lawyer.

“True,” he answered.

James E. Long, a lawyer who represented Mr. Hussain from 2002, when he was arrested, until 2006, refused to comment.

William C. Pericak, an assistant United States attorney in Albany who prosecuted Mr. Aref and Mr. Hossain, also would not comment about the informant. But after the convictions of the two men, he said, “You can’t put a percentage on how likely these guys would have been to commit an act of terrorism. But if a terrorist came to Albany, my opinion is that these guys would have assisted 100 percent.”

The man called Maqsood, before appearing in Newburgh, had first approached the Masjid Al-Noor mosque in nearby Wappingers Falls, according to members there. The imam and several board members said a man who called himself Maqsood started sporadically attending services there in 2007. He was flashy, they said, and bragged about his real estate business and properties. He drove a black Mercedes and always came alone.

Zubair Zoha, a former treasurer of the mosque, said the man asked him three times for the full list of members of the mosque, saying he wanted to approach potential customers. But he was largely ignored or dismissed.

He stopped coming, the members said, around June 2008.

It was then, according to the government’s court papers, that their informant struck up a relationship with Mr. Cromitie at the mosque in Newburgh, a set of dealings that would result in the bomb plot.

The imam in Newburgh, Salahuddin Mustafa Muhammad, said he was angry that the informant had associated his mosque with the scheme that had nothing to do with regular members. He condemned the plot, but questioned whether the men who were arrested would have committed to it had the informant not shown up.

Mr. Muhammad said he wondered whether he should have done anything differently once he had suspicions about the man named Maqsood.

“How do you go to the government about the government?” he asked.

Colin Moynihan, Nate Schweber and Karen Zraick contributed reporting.