US-Canada border crossings near Niagara Falls were closed Wednesday as terrorism investigators responded to a vehicle explosion at Rainbow Bridge, local and state authorities said. The cause of the incident was not immediately clear but New York Governor Kathy Hochul said state police and the FBI Joint Terrorism Task Force were monitoring all points of entry into the state. Ron Rienas, of the Buffalo and Fort Erie Public Bridge Authority, told ABC News all four bridges connecting the two countries over the Niagara River have been closed.