BROOKLYN - A convicted terrorist took the stand today to testify against a former friend who stands accused of plotting to bomb the New York City subway system. The federal trial of Adis Madunjanin began today in Brooklyn. Prosecutors say he was the catalyst who convinced two high school friends to begin training with al-Qaida. Madunjanin and two others, Zarein Ahmedzay and Najibullah Aziz, allegedly took a plane from Newark to Pakistan in 2008, where they were instructed by al-Qaida operatives to carry out a suicide mission. Ahmedzay testified against Madunjanin today, saying he led them to become "very radical" as they prepared to kill themselves and others for al-Qaida. Prosecutors say the men fed al-Qaida information about public transportation, including the Long Island Rail Road. Federal authorities say a fourth man connected with the foiled plot also struck a plea deal with prosecutors and is expected to testify in coming days. That man, Bryant Neal Vinas, was born and raised in Patchogue, and at one point planned to bomb a train at Penn Station, officials say.