Officials said while Rao's son Vivek Papisetty was first summoned to join investigations last week, after searches were carried out against him and his family on December 21, and now fresh reminders have again been sent to him in this regard.
They said Vivek has informed the Investigating Officer (IO) of the case that he would join the probe soon and has cited his wife's illness as the reason for not being able to do so till now.
Reddy was arrested by CBI last week.
"Vivek has said he would be joining I-T investigations in a day or two. His questioning is vital in the case," they said, adding that three other associates searched in the case are also being questioned by the department.
More From This Section
I-T officials had earlier claimed that Vivek had "admitted to a total unaccounted income of about Rs 17 crore", a day after I-T raids were carried out at the top bureaucrat's house and other premises linked to his kin.
The Tamil Nadu government, on the very next day of raids on December 22, had replaced Rao from the top bureaucratic post and had appointed senior IAS officer and Additional Chief Secretary Girija Vaidyanathan as the new Chief Secretary.
The I-T case emerges from the to biggest cash haul of new notes post demonetisation after the department raided some sand mining operators of Tamil Nadu.
Over Rs 142 crore unaccounted assets have been recovered so far by the department in this case apart from seizure of Rs 34 crore in new notes.