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WASHINGTON 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.

The report indicates that al-Qaida terrorists "in late September may have discussed targeting transit systems in and around New York City. These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger railsystems," according to the document.

"We have no specific details to confirm that this plot has developed beyond aspirational planning, but we are issuing this warning out of concern that such an attack could possibly be conducted during the forthcoming holiday season," states the warning, which is dated Tuesday.

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

Intelligence and homeland security officials are working with local authorities to try to corroborate the information "and will continue to investigate every possible lead," the memo says.