A massive manhunt by security and intelligence agencies is underway in the United States for three men believed to be planning major terror attacks in the country. Secretary of State Hillary Clinton said that in New York at a function that cities from New York to Mumbai are still under threat. She said that there is a credible and specific” information that US citizens or people with American travel documents are on a mission to explode a car or vehicle bombs in New York or Washington. The alarm was sounded on the basis of tip off which came to US agencies in mid week from a CIA informer in Pakistan's troubled northwest. According to tip off message a three member team has been dispatched by new al-Qaeda chief Egyptian Aiyman al-Zawahiri, who wants to mark the 10th anniversary of the 9/11 attacks by spectacular strikes on the US soil as a revenge for killing of Osama bin Laden. Late Wednesday, the officials received details which they consider specific. Jumping into action, American law enforcement authorities increased security measures at airports, nuclear plants and train stations. At New York's main Penn station, transit authorities and state police are seen carrying assault rifles and wearing bullet proof vests as they watch crowds of commuters. Police check posts have sprouted at all major road intersections with sniffer dogs carrying out checks on cars and other vehicles.