[TOKYO] An independent investigation of Toshiba Corp's accounting irregularities is looking at the role played by top officials, people familiar with the matter said, a move that could force a management overhaul at the Japanese conglomerate. As the probe has widened from looking into accounting errors at Toshiba's infrastructure business to checking books for a broad range of operations, investigators have interviewed upper management and examined internal e-mails in an effort to find the root cause of the irregularities, the people said. Toshiba has been unable to close its books for the past financial year and seen its market value slide by a quarter or US$4 billion due to the accounting scandal - which has become Japan's biggest since camera maker Olympus admitted in 2011 it used M&A deals to conceal investment losses.