The informant, a 19-year-old American citizen of Bangladeshi descent, was recruited by the NYPD recently as part of an expansive intelligence-gathering program the department launched after the al-Qaeda attacks of September 11, 2001. His assignment was to make contact with suspected Islamic extremists to try to determine if they had any inclinations to engage in violence, the source said.

On October 2, however, the informant, whom the source did not name, posted a message on his personal Facebook page exposing himself as an informant to people he had been in contact with. He declared that he had quit as a police informant.

I was jus (sic) of pretending to be friends with ya cuz I honestly thought i was fighting terrorism, but let's be real, it's all a f...king scheme, the informant wrote, according to the source. It was all about the money, he added.

The source said that the informant was not involved in an investigation that led to the arrest of a Bangladeshi man last week in connection with an alleged scheme to bomb the New York Federal Reserve Bank in Lower Manhattan.

New York law enforcement sources have said the NYPD has used foreign-born confidential informants to uncover several alleged plots by militants, including one involving a possible attack on a subway station at Herald Square and another involving alleged plans to kill US soldiers returning to New York from Afghanistan and Iraq.

NYPD Deputy Commissioner Paul Browne said candidates to join the force as sworn officers must be US citizens. But he said 20 per cent of the department's recruit classes were foreign-born.