According to the prisons department, investigation agencies other than NIA, customs and the Enforcement Directorate cannot interrogate Swapna directly.
The Crime Branch can commence an investigation only after registering an FIR and this means Swapna has to be interrogated to get details about the clip.
A three-member advisory board comprising High Court judges was formed to review COFEPOSA detention in the case.
A Special Investigation Team of the crime branch has been entrusted to conduct the preliminary enquiry, police sources said.
On Thursday, the prisons department conducted a preliminary inquiry and submitted a report to Rishi Raj Singh.
Swapna audio clip row: Prison dept submits report to DGP; Police likely to receive legal advice soon
The report also suggested police probe to find out the source of the clip as the preliminary probe ruled out the possibility of recording the clip at Attakulangara women's prison.
Swapna Suresh was heard saying that the investigation officers offered her to become an approver in the case if she falsely claimed chief minister's link in the case.
Customs is set to interrogate Sarith at Viyyur Central Jail and Swapna at Attakulangara Women's Prison on Wednesday.
The team from Kochi reached the prison at 10 am and interrogation began at 10:30 am to quiz Swapna for the second time in jail.
Swapna came to know about the plan from Khalid himself on July 4. Khalid is an Egyptian national who worked in the Consulate.
Around 9pm, they left the resort and reached Ernakulam past midnight.
Swapna is being interrogated at Attakulangara women's prison while Sandeep is being grilled at Poojappura central prison
Kerala LIFE Mission scam: M Sivasankar arraigned as fifth accused, Vigilance interrogates Swapna Suresh at jail
Vigilance officers had collected the bank account details of Swapna Suresh. She has accounts at Axis Bank and SBI in Thiruvananthapuram.
NIA had told the court that about 2,000 GB of data was recovered from the digital devices seized from Swapna and Sandeep Nair.
The row erupted after Swapna Suresh, a key accused in the gold smuggling case, revealed to the NIA that she had received Rs 1 crore as commission from Unitac, the builder