Mixmotive/iStock(TUCSON, Ariz.) -- One Drug Enforcement Administration agent was killed and two other officers were injured in a shooting on an Amtrak train that was stopped in Tucson, Arizona, authorities said. Agents boarded the train Monday morning to perform a routine check for illegal guns, money and drugs, after receiving a tip from Amtrak. Prior to boarding, they were given a list of names of the passengers on board -- two of whom were the suspects, Devonte Okeith Mathis and D.T., according to a criminal complaint against Mathis. Once on the train, officers encountered Mathis and D.T. o...