By Sophie Knight and Nathan Layne TOKYO (Reuters) – Two years before Mt. Gox filed for bankruptcy, a half dozen employees at the Tokyo-based bitcoin exchange challenged CEO Mark Karpeles over whether client money was being used to cover costs, according to three people who participated in the discussion. The question of how Mt. Gox handled other peoples money – the issue raised by staff in the showdown with Karpeles in early 2012 – remains crucial to unraveling a multi-million dollar mystery under examination by authorities in Japan. The still-unresolved issue has thrown a spotlight on how Mt. Gox functioned as a hybrid between an online brokerage and an exchange. Essentially, the more than 1 million traders who used Mt. Gox at its peak had entrusted a 3-year-old firm to hold their money safely until they decided to cash out.