Truck owners and truck rental agencies in the U.S. must be "vigilant" about the increasing frequency of "ramming attacks" by terrorists using vehicles as killing machines, according to a new Transportation Security Administration report obtained exclusively by NBC News. The unclassified report warns owners, operators and rental agencies to protect their vehicles from theft. Entitled "Vehicle ramming attacks: Threat landscape, indicators and counter measures," the report was sent Tuesday to local law enforcement agencies and private companies working with the Department of Homeland Security. The report notes that in the past three years, at least 173 people have been killed...