Federal prosecutors on Wednesday accused a 21-year-old Bangladeshi man of conspiring to blow up the Federal Reserve Bank of New York, saying that he tried to detonate what he believed was a 1,000-pound bomb in a van he parked outside the building in Lower Manhattan.

But the entire plot, which began after the man, Quazi Mohammad Rezwanul Ahsan Nafis, came to the United States in January, unfolded under the surveillance of the Federal Bureau of Investigation and the New York Police Department as part of an elaborate sting operation, according to court papers.

Nafis told an FBI informer in July that he had overseas connections to Al Qaeda, according to a criminal complaint unsealed Wednesday afternoon in Federal District Court in Brooklyn.

Nafis, the complaint said, had been trying to recruit people to form a terrorist cell and sought out Qaeda contacts to help him carry out an attack. One of the people he tried to recruit was the FBI informer, who later introduced him to an FBI undercover agent, according to the complaint.

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