Police bolstered security in subways and trains Wednesday after the government warned that al-Qaida suicide bombers were contemplating an attack on New York's mass-transit systems 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 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 person briefed on the matter, speaking to The Associated Press 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, which are particularly busy with Thanksgiving holiday travelers.

A federal law enforcement official said 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."

The Big Apple's tightly packed passenger trains and subway cars have long been a source of concern for police officers - and a tempting target for would-be terrorists - but there is often disagreement as to how seriously authorities should take specific intelligence reports.

The city has more than 450 subway stations that handle millions of commuters every day.

A Pakistani immigrant was arrested and convicted 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; another aborted al-Qaida plot to destroy the Brooklyn Bridge in 2003; and a plot to bomb underwater train tunnels to flood lower Manhattan, which was broken up in 2006 by several arrests overseas.

Three years ago, authorities weighed 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.

NYPD spokesman Paul Browne said they have received an unsubstantiated report and as a result have "deployed additional resources in the mass transit system."

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.

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 "out of an abundance of caution going into this holiday season."

No changes are being made to the nation's threat level, or for transit systems at this time, he 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.