Jose Pimentel, 29, a US citizen born in the Dominican Republic, was arrested by New York police in November 2011 and has been portrayed by officials as a "lone wolf" terrorist inspired by Al-Qaeda.
He was initially indicted on five counts and could have been jailed for life if convicted faced. He is now expected to face 16 years behind bars when sentenced on March 25.
Pimentel, today pleaded guilty in the New York state supreme court to attempted criminal possession of a weapon in the first degree as a crime of terrorism.
He was accused of plotting to bomb police cars and post offices and kill US servicemen returning from Afghanistan and Iraq to protest the American military intervention in those countries.
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Pimentel, who allegedly was building a pipe bomb at the time of his arrest, had been under surveillance by New York police for about two years.
He initially pleaded not guilty to the charges. Terrorism cases are usually prosecuted in federal courts in the United States.
Pimentel's attorneys Susan Walsh and Lori Cohen had earlier described the case as "overreaching" and an example of an overzealous and wrongheaded effort to stamp out terror.
According to documents filed in court, Pimentel collected spare parts such as incendiary powder, pipes with drilled holes, electronic circuits, clocks and nails to build pipe bombs.
The prosecution says each of the components was proscribed in a step-by-step guide in Al-Qaeda's English-language Inspire Magazine on how to make a bomb designed to maximize casualties.