WASHINGTON – 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.

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 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.