WASHINGTON - (AP) - Federal authorities are warning lawenforcement personnel of a possible terror plot against the NewYork City subway and train systems during the holiday season, andpolice are beefing up security in preparation.

An internal memo obtained by The Associated Press says the FBIhas received a "plausible but unsubstantiated" report thatal-Qaida terrorists in late September may have discussed attackingthe subway system.

A person briefed on the matter, speaking to The Associated Presson condition of anonymity because of the sensitivity of theintelligence-gathering work, said the threat may also be directedat the passenger rail lines running through New York, such asAmtrak and the Long Island Rail Road, which are particularly busywith Thanksgiving holiday travelers.

A U.S. counterterror official, also speaking on condition ofanonymity because he was not authorized to do so publicly, saidsenior government officials have been briefed because the FBI veryrecently received credible information about possible attacks overthe holiday season, and authorities are particularly concernedabout this long holiday weekend.

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

Department of Homeland Security spokesman Russ Knocke said thewarning was issued as a routine matter, but added that there may bean increased police presence in New York and other large cities.

The internal bulletin says al-Qaida terrorists "in lateSeptember may have discussed targeting transit systems in andaround New York City. These discussions reportedly involved the useof suicide bombers or explosives placed on subway/passenger railsystems," according to the document.

"We have no specific details to confirm that this plot hasdeveloped beyond aspirational planning, but we are issuing thiswarning out of concern that such an attack could possibly beconducted during the forthcoming holiday season," according to thewarning dated Tuesday.

NYPD spokesman Paul Browne said they have received anunsubstantiated report and as a result have "deployed additionalresources in the mass transit system."

While federal agencies regularly issue all sorts of advisorywarnings, the language of this one is particularly blunt.

Intelligence and homeland security officials are working withlocal authorities to try to corroborate the information "and willcontinue to investigate every possible lead," the memo says.

Rep. Peter King, the top Republican on the House HomelandSecurity Committee, said authorities "have very real specifics asto who it is and where the conversation took place and whoconducted it."

"It certainly involves suicide bombing attacks on the masstransit system in and around New York and it's plausible, butthere's no evidence yet that it's in the process of being carriedout," King said.

Knocke, the DHS spokesman, said the warning was issued "out ofan abundance of caution going into this holiday season."

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

"However, transit passengers in larger metropolitan areas likeNew York may see an increased security presence in the comingdays," Knocke said.

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