The CBI's investigation into the allotment of coal blocks has reached the doorstep of the Prime Minister's Office (PMO). The agency has asked TKA Nair, Adviser to Prime Minister Manmohan Singh, to appear before it to record his statement as a "witness". CBI recently examined two former PMO officials, IAS officer Vini Mahajan and IPS officer Ashish Gupta, who are learnt to have explained that the process of allocation had Nair at the helm.