Federal authorities are warning of a possible Al Qaeda-orchestrated terror plot against New York City subway and other transit systems during the holidays, FOX News has confirmed.

The potential threat — described in an internal FBI memo as "plausible but unsubstantiated" — does not extend beyond the New York City area, sources told FOX News. But commuters could see security tighten across the country.

A person briefed on the matter, speaking on condition of anonymity because of the sensitivity of the intelligence-gathering work, said the threat may also be directed at the passenger rail lines running through New York, such as Amtrak and the Long Island Rail Road. The threat surfaced on one of the busiest travel days of the year and when tens of thousands of tourists are in New York City for the Macy's Thanksgiving Day Parade.

The FBI and the Department of Homeland Security issued a joint bulletin late Tuesday night to local and state law enforcement authorities about the possible plot, a Homeland Security official told FOX News.

The report states that in late September, Al Qaeda may have talked about attacking transit systems in and around New York City during the holiday season.

"These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger rail systems," the bulletin says.

The FBI said it had no indication that any such plot was beyond the very earliest stages of planning.

"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," according to the memo.

Beth Maderal, 26, learned terrorists might be plotting a holiday attack as she stood on a Brooklyn subway platform with her mother waiting for the train to Manhattan. Maderal said any threat gives her pause, but she is not going to stop riding the subway because she depends on it so much.

"If you get scared that means they win," commuter Omid Sima said on the platform of the subway below Rockefeller Center. "There's always been terror warnings. I can't change my life because of that."

A source with close knowledge of the bulletin says the information has links to the tribal areas of Pakistan.

"Every resource is being brought to bear on corroborating this information," the source told FOX News.

FOX News' Catherine Herridge and The Associated Press contributed to this report.