Zazi, 26, was testifying for a second day at the trial of Adis Medunjanin in federal court in Brooklyn.
Prosecutors allege that Medunjanin, Zazi and another former high school classmate from Queens, Zarein Ahmedzay, formed a terror cell that posed one of the most ominous terror threats since 9/11.
Zazi has testified that during their 2008 trip to Pakistan, the three Americans met a top al-Qaida operative they knew only as Hamad. Authorities say Hamad was Adnan Shukrijumah, a Saudi still listed on an FBI website as a fugitive who plotted attacks for al-Qaida worldwide.
Hamad told the three that they were best suited for an operation on U.S. soil. He also mulled over potential targets with them, including the New York Stock Exchange, Times Square and an unspecified Walmart store, Zazi said.
The men ended up choosing the subway because "it's the heart of everything in New York City," Zazi said Wednesday. The purpose, he added, was "to make America weak."
In a later meeting in New York, the plotters decided to blow themselves up at three different locations at morning rush hour inside the Manhattan subway system during the month of Ramadan, Zazi said. Their hope was that "people would have a lot of fear," he said.
Medunjanin, 27, a Bosnian-born Muslim and naturalized U.S. citizen, has pleaded not guilty to conspiracy to use weapons of mass destruction, providing material support to a terrorist organization and other charges. He has denied he was ever part of an al-Qaida operation.
Authorities have portrayed Zazi as a homegrown terrorist who orchestrated the 2009 scheme to strap on suicide bomb vests and detonate them inside Manhattan subways. Both he and Ahmedzay pleaded guilty in 2010 and were jailed without bail after agreeing to become government witnesses in a bid for leniency.
After leaving Pakistan, Zazi relocated to the Denver area, where he used beauty supplies to try and cook up explosives in a hotel room and set out for New York around the time of the eighth anniversary of the Sept. 11 attacks. Once he suspected he was under surveillance, he aborted the mission and returned to Colorado, where he was arrested.