Photograph of police in the subway station for the 2006 No Pants Ride by icopythat on Flickr

Want some possible terror plot with your turkey? The AP reports, "Federal authorities are warning law enforcement personnel of a possible terror plot against the New York City subway system during the holiday season." This is based on an internal memo the AP got a hold of:

The FBI has received a "plausible but unsubstantiated" report that al-Qaida terrorists in late September may have discussed attacking the subway system.

Anti-terror agencies say they have no specific details to confirm the plot has moved "beyond aspirational planning," but are issuing the alert out of concern that an attack could come during the holiday season," warns the memo, which is dated Tuesday.

While federal agencies often issue all sorts of advisory warnings, the language of this one is particularly forceful.

Related: There was an alleged subway terror threat in October 2005--which some pointed out diverted attention from the 2005 mayoral debate Mayor Bloomberg declined to attend.

Update: ABC News reports the FBI calls the threat "plausible but unsubstantiated," which makes ABC News analyst--and former CIA intelligence officer--John Kiriakou think the information came from a walk-in to an embassy, "That's the kind of language we use when we can't back it up but we can't knock it down either."

The Department of Homeland Security says, "However, transit passengers in larger metropolitan areas like New York may see an increased security presence in the coming days. This includes uniformed and plain clothed behavior detection officers, federal air marshals, canine teams, and security inspectors."