According to a Fox News report, an internal FBI memo warned that such an attack is "plausible but unsubstantiated". A person briefed on the matter, speaking 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. The threat surfaced on one of the busiest travel days of the year and when tens of thousands of tourists are in New York City for the Macy's Thanksgiving Day Parade.
The FBI and the Department of Homeland Security issued a joint bulletin late Tuesday night to local and state law enforcement authorities about the possible plot, a Homeland Security official told Fox News.
The report states that in late September, Al Qaeda may have talked about attacking transit systems in and around New York City during the holiday season.
"These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger rail systems," the bulletin says.
The FBI said it had no indication that any such plot was beyond the very earliest stages of planning.
"We have no specific details to confirm that this plot has developed beyond aspirational planning, but we are issuing this warning out of concern that such an attack could possibly be conducted during the forthcoming holiday season," states the warning, which is dated Tuesday.
While federal agencies regularly issue all sorts of advisory warnings, the language of this one is particularly blunt.
Intelligence and homeland security officials are working with local authorities to try to corroborate the information "and will continue to investigate every possible lead," according to the memo.