A mysterious image threatening the return to New York of Al Qaeda has appeared on a handful of Arabic websites, prompting the New York police department and federal authorities to investigate.

The chilling graphic, which looks like a poster for an upcoming movie, includes the English-language taunt, "Al Qaeda Coming Soon Again in New York." NYPD spokesman Paul Browne said the image appeared in the "artwork and design" section of a murky site used as a forum for terror talk.

"The NYPD Intelligence Division’s cyber unit is investigating the origin and significance of the graphic below which appeared today on few Arabic-language Al Qaeda forums that remain online at the moment," Browne said.

While city and federal authorities said there is no known threat behind the posting, they are trying to determine who is behind it. An FBI spokesman said the bureau is conducting a probe of its own.

"The FBI's Joint Terrorism Task Force is aware of the posting and investigating its authenticity and origin," said J. Peter Donald, spokesman for the FBI's New York Field Office. "The FBI takes all threats seriously and at this time there is no specific or credible threat to New York."