The Central Bureau of Investigation (CBI) is likely to question former prime minister Manmohan Singh in AgustaWestland case, said a report. Along with him, the CBI may also question former principal secretary T K A Nair, former national security adviser MK Narayanan, former CBI director Ranjit Sinha and former special director Salim Ali. This follows the arrest of former Indian Air Force chief S P Tyagi in connection with the case.