New York City is on high alert after a terror warning from the Department of Homeland Security.

The FBI received a "plausible but unsubstantiated" threat al-Qaida was considering a terrorist attack on transit systems in and around New York City over the Thanksgiving holiday. Additional police forces have been deployed to watch the subways.

According to the FBI, al-Qaida discussed a plan in September that would involve placing suicide bombers or explosives on trains.

The nation's threat level remains at yellow.

Feds warn of terror plot against NYC subways