Federal law enforcement sources tell ABC News they had been on the alert for a possible shoe bomber when a federal air marshal opened fire at the Miami International Airport today. In today's incident, an agitated passenger claimed to have a bomb in his backpack was shot and killed by a federal air marshal, officials said. No bomb was found. Officials say a 50-year old Egyptian man was stopped six days ago at New York's John F. Kennedy airport. Sources say he had a suspicious pair of shoes that tested positive five times for the explosive substance TATP on the...