Homeland Security officials are warning of the possibility of imminent retaliatory terrorist attacks on U.S. and western targets following the death of Osama bin Laden, an unreleased document issued overnight and reviewed by the Investigative Project on Terrorism shows.

The internal U.S. government document, "Possible Implications Regarding the Death of Usama Bin Ladin" was prepared by intelligence analysts at DHS and at the FBI.

"The Intelligence Community (IC) assesses the death of al-Qaida leader Usama Bin Ladin could result in retaliatory attacks in the Homeland and against US and Western interests overseas," the document says. "Attacks might originate with al-Qaida core elements in the tribal areas of Pakistan, with one of their affiliates overseas, and/or with individuals in the homeland sympathetic to the cause but lacking a formal group association."

The document further stated: "We have no indications of advanced al-Qa'ida Core plotting efforts in the Homeland, but the case of now-detained al-Qa'ida operative Najibullah Zazi—who, along with two associates, planned to attack the New York City subway in 2009 using homemade explosives—demonstrates that unidentified operatives could advance plotting in the homeland."

Other intelligence and counter-terrorism sources told the Investigative Project on Terrorism that security has been ramped up in major cities across the United States, especially focusing on possible targets such as mass transportation, government buildings, and critical infrastructure. Overseas, additional security has been added to US embassies and consular offices in numerous countries in the Middle East, Persian Gulf, Asia, Africa and Europe.

Military sources told the Investigative Project on Terrorism that they were definitely bracing for imminent retaliatory attacks on US military bases and installations in Pakistan, Afghanistan and Iraq where there are large numbers of al-Qaida members and supporters.