Federal prosecutors and a former Metropolitan Mortgage & Securities Co. executive accused of deceiving auditors are negotiating a possible plea agreement. Such a development with Thomas Turner, who worked as the No. 2 executive at the financial conglomerate, would net the first conviction in the massive accounting fraud that wiped out the savings of thousands of regional investors. And it may offer insight into whether C. Paul Sandifur Jr., the colorful and controversial former CEO and chairman of Metropolitan, will be charged with crimes.