Former telecom minister A Raja was questioned for close to nine hours by the Central Bureau of Investigation (CBI) on Friday to establish his alleged role in irregularities in the 2G spectrum allocation.
The interrogation came 14 months after the agency filed its first information report in the case.
Sources said the CBI is trying to establish possible financial links between Raja and the companies that gained from the allocation of 2G spectrum.
The allocation of spectrum on a first-come-first-served basis is estimated to have cost the country Rs 1.76 lakh crore.
Raja flew in from Chennai on Wednesday, arrived at the CBI headquarters at 10.30am and was there till late evening.
He was asked about his alleged role in advancing the dates for allocation of spectrum licences and whether his relatives were connected to a few of the companies that were fronting for other telecom firms that got the licences between September 2007 and January 2008.
Raja, sources said, was grilled to establish if he was part of a "criminal conspiracy" behind the spectrum allocation at undervalued prices.
"He was questioned about the decision-making process that resulted in the allocation of the unified access service licences and the large number of incriminating documents, receipts and cyber evidence we have gathered against the key accused," said a source.
Several questions were based on revelations made by former Telecom Regulatory Authority of India chairman Pradeep Baijal about their joint role in the allocation of licences.
Raja, who emerged from the CBI headquarters at 7.30pm, refused to comment in view of the ongoing probe but said he would "cooperate with the CBI".
Two weeks ago, the CBI had raided the homes of Raja and lobbyist Niira Radia, and seized Radia's laptop and Raja's personal documents, including a diary.
These could be key to the investigation, the sources said.