(Reuters) - An internal NBC News probe has determined a "seasoned" producer was to blame for a misleading clip of a 911 call that the network broadcast during its coverage of the Trayvon Martin shooting, according to two sources at the network. NBC News brass interviewed more than half a dozen staffers during its investigation of the misleadingly edited 911 call placed by George Zimmerman just before he shot the unarmed Florida teenager, said the sources, one of whom is an executive at the network. The clip aired on the network's flagship "Today" morning show last week. The edit made...