Official sources said the questioning would form the basis for summoning former Telecom Minister A Raja to the CBI headquarters.
Sources said the agency questioned R K Chandolia, Raja's former personal secretary, Siddhartha Behuria, then DoT secretary, K Sridhar, then Member, Telecom Commission, A K Srivastava, then Deputy Director General, Access Service Wing, DoT, and Raja's business partner Sadiqui Batcha, Managing Director of Chennai-based Greenhouse Exports.
The agency is also examining a a diary with various details, which is said to have been recovered during the searches conducted by the investigating agency on Wednesday from the residence of Raja.
The diary, according to sources privy to the probe, has some details pertaining to the allocation of 2G spectrum among other details.
Sources said CBI sleuths spent considerable time in checking the entries made in the diary which consist of names of some politicians and corporate honchos.
Raja was forced to resign as minister on November 14 in the wake of CAG's report that the rates at which 2G spectrum was allotted resulted in a possible loss to the exchequer to the tune of Rs 1.76 lakh crore.
Besides being accused of undervaluing 2G spectrum, Raja has also been blamed for circumventing the rules to favour companies that did not meet the eligibility criteria for getting spectrum licences.
The CBI had registered a case on October 21, 2009 against unknown Department of Telecom officials and private persons and companies for allegedly causing loss of thousands of crores of rupees to the exchequer.