WASHINGTON (AP) -- Federal authorities are warning police of a possible terror plot against the New York City subway and train systems during the holiday season, prompting local officials to beef up security at stations. 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.
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 U.S. counterterror official, also 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.
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 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.
The Big Apple's tightly packed passenger trains and subway cars have long been a source of concern for cops - and a tempting target for would-be terrorists - but there is often disagreement as to how seriously authorities should take specific intelligence reports.
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.
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.
"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.
Knocke, the DHS spokesman, 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.
"However, transit passengers in larger metropolitan areas like New York may see an increased security presence in the coming days," Knocke said.
The increased personnel could include uniformed and plainclothes "behavior detection" officers, federal air marshals, canine teams, and security inspectors, Knocke said.
Associated Press Writers Lara Jakes Jordan in Washington and Colleen Long in New York contributed to this report.
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