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.

A federal law enforcement official said that there's no indication that anyone involved in the planning is in the United States. That official also spoke on condition of anonymity because it involved intelligence-gathering.

While law enforcement stepped up patrols around subways and trains, many commuters around the city were unfazed by the news and had not even heard of the threat.

"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."

"I could get hit by a truck, too, so it doesn't bother me," said commuter Paul Greenwald of Long Island. "To me, it's just fear tactics."

The vulnerability of the city's tightly packed passenger trains and subway cars has long been a source of concern for cops — and target for would-be terrorists. The city has more than 450 subway stations that handle millions of commuters every day.

There is often disagreement as to how seriously authorities should take specific intelligence reports related to threats against mass transit — an issue that took on greater urgency following transit bombings in Madrid and London in 2004 and 2005 that killed a combined 243 people.

A Pakistani immigrant was arrested and convicted in New York for a scheme to blow up the subway station at Herald Square in 2004. There was also a planned cyanide attack on the subways by Al Qaeda operatives that authorities say was called off in 2002, and a plot to bomb underwater train tunnels to flood lower Manhattan. That plot was broken up in 2006 by several arrests overseas.

Three years ago, authorities weighed the seriousness of reports that bombers might try to use baby strollers to bring explosives into city trains. Many security officials later concluded that was a false alarm.

Bruce McIndoe, president of iJET Intelligent Risk Systems, which advises companies on travel safety, said transportation is always a "hot" area for terror concerns, but individual warnings don't mean terrorists have the capability to pull off such an attack.

McIndoe said the real message of the federal bulletin to local police is: "We're picking up some chatter. We have nothing to substantiate it, so it's better to be safe and step it up a bit."

The increased personnel could include uniformed and plainclothes "behavior detection" officers, federal air marshals, canine teams, and security inspectors, Knocke said.

FBI spokesman Richard Kolko confirmed only that his agency and the Homeland Security Department issued a bulletin Tuesday night to state and local authorities, and the information is being reviewed.

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.

"My whole life is riding the subway," the dance and yoga instructor said. "If it's open, I am going to ride it. ... If it's unsafe, they'll close it. The city wouldn't put me in danger."

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