He was already aboard Emirates Flight 202 when he was identified by the Department of Homeland Security’s United States Customs and Border Protection, according to a joint statement issued by the office of Preet Bharara, the United States attorney for the Southern District of New York; the F.B.I.; and the New York Police Department.

Officials called the plane back, the airline said. All of the passengers were taken off, and they, their luggage and the Boeing 777 were screened before the flight was allowed to depart, about seven hours late, at 6:29 a.m. It was not known when Mr. Shahzad bought his ticket.