NEW YORK -- The Secret Service, FBI and New York Police Department are investigating what sources tell CBS News is a threatening piece of mail sent to Donald Trump's son, Eric Trump. The mail, sent to Eric Trump at his posh Manhattan residence at 100 Central Park South, also known as Trump Parc East, was opened by his wife Thursday. A source close to the investigation said the letter had a Massachusetts postmark and warned that if Donald Trump doesn't withdraw from the race for the Republican presidential nomination -- paraphrasing -- harm will come to the kids. Investigators in...