The graphic contained a picture of the Manhattan skyline superimposed with a Hollywood-style caption that says: "AL QAEDA - coming soon again in New York."
It was posted on Monday by a site called the Ansar al Mujahiddin Arabic Forum, or AMAF, a militant web forum which allegedly has close connections to the Afghan Taliban and a key militant leader in Jordan.
At the same time, two Internet forums authorities believe that official Al Qaeda sanctions have been down for nearly two weeks, said Evan Kohlmann, an expert who monitors militant websites for government entities and private businesses.
A spokesman for the Federal Bureau of Investigation (FBI) office in New York said the Joint Terrorism Task Force was investigating whether the posting was authentic and that while every threat is taken seriously "there is no specific or credible threat to New York."
New York Police Department Chief spokesman Paul Browne said his officers were "investigating the origin and significance of the graphic ... which appeared today on a few Arabic-language al Qaeda forums that remain online at the moment."
Mr Browne noted, however, that the graphic was posted in a section of the forum labeled "art and design". Mr Kohlmann said that this raised questions as to whether the messages really constituted a serious threat. "That's not where an important threat would be posted," he said.
A US intelligence official said federal agencies would examine the AMAF graphic to "evaluate" its significance.
Al QAEDA FORUMS DOWN
The US official said that government agencies also were aware of the interruptions in traffic on the two main websites which purportedly have official sanction from Al Qaeda.
Mr Kohlmann identified these as Arabic-language forums called Shamukh and Al Fidaa. He said that they were recognized as official Al Qaeda forums because they were where the group's couriers first post "official" videos produced by what remains of Al Qaeda's core leadership and its affiliates, such as Al Qaeda in the Arabian Peninsula, based in Yemen.
Mr Kohlmann said that in the past, the two official websites had sometimes gone offline, but never for the length of time have they lately been down. He said that usually when the sites went offline some kind of explanation was posted.
But he said that this time, no explanation appeared until Monday, when a message on one of the sites said that it would be up and running again shortly after a few more "tweaks."
Mr Kohlmann said that the outages began roughly around the time of two potentially significant recent events: the arrest in Spain of a suspect who some investigators believe was an administrator for some militant websites, and the shootout in Toulouse, France in which Mohammed Merah, a militant who shot dead a rabbi, three children and three French soldiers, was himself killed by police.
Mr Kohlmann said speculation in the online world was that the forums had gone offline because they were attacked by government or non-official hackers of some kind.
The outages of Al Qaeda-linked sites were first reported by the Washington Post.
A person familiar with US government monitoring of militants said that because US authorities have erected legal barriers severely restricting the launching of offensive cyber-attacks by US agencies, it was unlikely the US government played a role in the militant websites' current outages.
(Copyright Thomson Reuters 2012)