Federal prosecutors have revealed that there was a fourth man in the United States connected to a 2009 al Qaeda-sanctioned plot to blow up subway lines in New York City.

Officials last week quietly revised the indictment against one of the conspirators, Adis Medunjanin, alleging that he — along with former high school classmates Najibullah Zazi and Zarein Ahmedzay — tried to recruit someone identified only as “John Doe” to travel to Pakistan “to wage violent jihad.”

It’s the first time prosecutors indicated there was a fourth person in the United States linked to what may have been the most legitimate terror plot since 9/11.

Now Medunjanin’s attorney, Robert Gottlieb, wants Brooklyn federal prosecutors to unveil the identity of the mystery man before he turns up as a possible witness at a trial set for March.