Federal authorities are warning law enforcement personnel of a possible terror plot against the New York City subway system during the holiday season.

An internal memo obtained by The Associated Press says the FBI has received a “plausible but unsubstantiated” report that al-Qaida terrorists in late September may have discussed attacking the subway system.

FBI spokesman Richard Kolko confirmed only that his agency and the Department of Homeland Security issued a bulletin last night to state and local authorities, and the information is being reviewed. (AP)