NEW YORK U.S. authorities have warned of a possible al Qaeda threat to transit systems in and around New York City, a Homeland Security official said on Wednesday.
New York police said they were increasing security in response to the warning but said this was "in an abundance of caution." The warning comes at the start of the Thanksgiving holiday weekend, one of the busiest U.S. travel periods of the year.
"The New York City Police Department is aware of an unsubstantiated report indicating that al Qaeda terrorists discussed targeting mass transit in New York City and the vicinity," Deputy Police Commissioner Paul Browne told Reuters.
Homeland Security Department spokesman Russ Knocke said the warning was issued on the basis of "plausible but uncorroborated information" and that al Qaeda may have discussed such attacks in late September.
Knocke said Homeland Security and the FBI had passed on the warning to state and local officials on Tuesday but there was no specific information to confirm that the plot had developed "beyond aspirational planning."
The warning was issued as a routine matter and no adjustments were being made to the nation's threat level.
New York City's Metropolitan Transportation Authority said it was aware of "threats against transit properties during the holiday season" and was working closely with officials to increase police presence throughout the sprawling bus and rail system.
Browne said: "In an abundance of caution the NYPD has deployed additional resources in the mass transit system."
New York has remained on an orange alert -- the second highest level, below red -- since the September 11, 2001, attacks that destroyed the World Trade Center. The rest of the United States is on yellow alert, one level below orange.
In October 2005, New York City's subway system was on high alert after detainees in Iraq were thought to be plotting a bomb attack. but the threat was later found to lack credibility.
In August 2007 police stepped up security throughout Manhattan and at bridges and tunnels in response to an unverified Internet report that al Qaeda might be plotting to detonate a dirty bomb in the city.
(Additional reporting by Deborah Charles in Washington; editing by Philip Barbara)