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.
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.
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.
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-Qaida 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, let's get through the holiday season and get on to the next thing."
While federal agencies regularly issue all sorts of advisory warnings, the language of this one is particularly blunt.
The internal bulletin says 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 rail systems," 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," according to the warning dated Tuesday.
A U.S. counterterror official, speaking on condition of anonymity because he was not authorized to do so publicly, said senior government officials have been briefed because the FBI very recently received credible information about possible attacks over the holiday season, and authorities are particularly concerned about this long holiday weekend.
Rep. Peter King, the top Republican on the House Homeland Security Committee, said authorities "have very real specifics as to who it is and where the conversation took place and who conducted it."
"It certainly involves suicide bombing attacks on the mass transit system in and around New York and it's plausible, but there's no evidence yet that it's in the process of being carried out," King said.
Department of Homeland Security spokesman Russ Knocke said the warning was issued as a routine matter, but added that there may be an increased police presence in New York and other large cities. 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. 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.
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."
Associated Press Writers Eileen Sullivan and Lara Jakes Jordan in Washington and Colleen Long, Verena Dobnik, Deepti Hajela, Adam Goldman and Tom McElroy in New York contributed to this report.