When Sanmina-SCI Corp. announced last fall that it had improperly backdated “most” of its stock options since 1997, it publicly blamed the wrongdoing on two former executives.

But behind closed boardroom doors, said people briefed on the company’s internal probe, a third man was fingered: Wilson Sonsini Goodrich & Rosati partner Christopher Mitchell, who had been the company’s outside counsel since the mid-1990s.