AP Photo/Kathy Willens, File

A New York City police officer keeps watch on a platform at the Times Square subway station during an evening rush hour. The FBI and the Homeland Security Department issued a bulletin Tuesday night, Nov. 25, 2008 to state and local law enforcement authori

WASHINGTON
-- Federal authorities are warning law enforcement personnel of a possible terror plot against the New York City subway and train systems during the holiday season, and police are beefing up security in preparation.

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.

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.

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.