Americans were warned Thursday of a “specific, credible but unconfirmed” threat of a terrorist attack planned to coincide with Sunday’s tenth anniversary of the 9/11 hijackings. New York police have been put on high alert ahead of the milestone date, with shifts extended and the number of officers on the street increased. According to ABC News, investigators are hunting for three men, one an American citizen, sent to the U.S. to carry out a “vehicle-borne attack against Washington, D.C. or New York.” Rumours of an anniversary attack have swirled for months. Documents found in Osama bin Laden’s compound suggest the late al-Qaeda leader hoped to bomb a train or fly a small plane into a public event ten years after he toppled the twin towers of the World Trade Center. For now, Sunday’s planned memorial at the site of the attacks will go on as scheduled.