Federal investigators in the US are turning to thousands of taped phone conversations as they build criminal cases related to the multibillion-dollar trading loss at JPMorgan Chase's London-based chief investment office, zeroing in on calls in which employees openly discussed how to value the troubled bets, the New York Times reports. But even if they decide to bring charges, prosecutors face an uphill battle to prove employees intentionally masked losses, as traders are sometimes allowed to estimate the value of derivatives holdings when market prices are not readily available.