New York City police stepped up patrols in the city's subways and trains Thursday after the federal government warned of a potential Al Qaeda attack during the holidays.

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.

NYPD spokesman Paul Browne said police have received the report and as a result have "deployed additional resources in the mass transit system." Browne wouldn't get into details on how many extra personnel would be deployed.

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.

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