The former lead prosecutor in the Enron trial was retained by San Bernardino County on Tuesday to lead an investigation into County Assessor Bill Postmus. The board approved a $250,000 contract with attorney John C. Hueston, a prosecutor in the case against Enron executives.  John Hueston of the Los Angeles-based law firm Irell & Manella LLP will investigate the county’s options in removing Postmus from office.  Postmus faces felony charges of drug possession of methamphetamine and drug paraphernalia at his home and is under investigation by the District Attorney’s office.  Assistant Assessor Dennis Draeger has been overseeing the County’s Assessor’s office in Postmus’s absence.