NEW YORK — Federal prosecutors accused a 21-year-old Bangladeshi man Wednesday 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 plot, which began ­after the accused man, Quazi Mohammad Rezwanul Ahsan Nafis, came to the United States in January, unfolded under the surveillance of the FBI and the New York Police Department as part of an elaborate sting operation, according to court papers.