CBI sources said agency will soon call Nair to record his statement as witness in connection with alleged irregularities in the allocations made during 2006 to 2009.
In a related move, CBI has asked former Coal Secretary H C Gupta, who recently resigned as member of Competition Commission of India, to appear before it for questioning as an accused in the case on Thursday.
The agency is likely to question him about the clearances for the allocation of coal blocks given by Screening Committee which was chaired by him on several occassions.
Meanwhile, the agency has recorded the statements of two officials--Vini Mahajan and Ashish Gupta--who were posted in the Prime Minister's Office during 2006 and 2009, the sources said, adding they had been questioned as witnesses in the case.
During that period, the Prime Minister was handling the charge of Coal Ministry.
Vini Mahajan, a 1987-batch IAS officer from Punjab cadre, was a Director with the PMO during the period and has now been repatriated back to the state where she is Principal Secretary level officer.
Ashish Gupta, an IPS-officer of 1989-batch from Uttar Pradesh cadre, who was the vigilance officer in the PMO, has also gone back to his cadre.
HC Gupta being the Coal Secretary was the de-facto Chairman of the Screening Committee which allocated coal blocks during 2006-09, the sources said.
The working of the screening committee has come under the scanner of the agency as CBI has accused "unnamed officials of the screening committee", in its latest FIR against Congress MP Naveen Jindal and former Minister of State Dasari Narayan Rao, of being part of a criminal conspiracy.
The sources said the committee did not follow the mandatory procedures and did not carry out proper background checks while allocating coal blocks to the companies.
In its status report filed on March 8, the CBI had said that the coal blocks allocation during 2006-09 was done without verifying the credentials of companies which allegedly misrepresented facts about themselves and no rationale was given by the Coal Ministry in giving coal blocks to them.
CBI has so far registered 12 FIRs in the matter.
Last month, CBI had received a flak from the Supreme Court for sharing its status report in the coal scam with the then Law Minister Ashwani Kumar and senior officials of the PMO and the Coal ministry and also incorporating changes suggested by them.
The apex court said that sharing of information with the government about the probe into the scam has "shaken the entire process" and asked CBI not take instructions from "political masters" on their probe.