"60 Minutes" investigates mysterious death of Argentine prosecutor Alberto Nisman

The Argentine government is calling for "commitment, truth and justice" after the mysterious death of Nisman. On Thursday, Nisman's ex-wife said he was murdered as he was preparing to reveal evidence against Argentina's government and president. "60 Minutes" correspondent Lesley Stahl joins "CBS This Morning" to give a preview of her report. She spoke to the employee who gave Nisman the gun used in the killing.