Official sources said the Directorate has recently started fresh questioning of representatives of certain telecom firms after analysing thousands of pages of documents submitted by them during the first interaction.
The documents submitted by the firms pertained to their income tax returns, shareholding patterns, subsidiaries and funding routes, the sources said.
The ED had registered a case under the Prevention of Money Laundering Act (PMLA) and is working in close cooperation with the CBI which is also investigating the scam.
Sources said the fresh questioning will focus on funding routes and their subsidiaries in tax haven countries like Mauritius.
The Directorate will also look into the stake sellout deals carried out by certain firms after bagging the spectrum.