A warning was also issued for the New York subway in October 2005
Federal authorities in the US have warned local police of a possible al-Qaeda plot to target New York's subway.
There was "plausible but uncorroborated" information that al-Qaeda may have discussed a plot in late September, an official said.
He said the warning was being issued out of an "abundance of caution" ahead of the holiday season.
Thursday marks Thanksgiving in the US, seen as the beginning of a holiday season that ends with New Year.
A leaked memo obtained by the Associated Press news agency said al-Qaeda "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," it said.
But Homeland Security spokesman Russ Knocke said neither his agency nor the FBI had any information to suggest that the plot has developed "beyond aspirational planning."
There were no changes to national threat levels, though more security officials might be deployed in large cities, he said.