Mr Rao was earlier summoned by the investigating team to appear before it for questioning in connection with the racket.
Meanwhile, an official heading the SIT probe asked people not to judge the investigation process without having complete knowledge about it.
He said the allegations that the probing officers are not doing their duty properly "amount to defamation".
The special investigation team (SIT) earlier summoned around 10 personalities, including actors from the Telugu film industry, for questioning in connection with the racket.
Telugu actor Pallapollu Navdeep was yesterday questioned by the SIT officials for over 10 hours.
The SIT had issued notices to all of them under the Narcotic Drugs and Psychotropic Substances (NDPS) Act after the racket for supplying high-end drugs such as LSD and MDMA in and around Hyderabad was busted on July 2.
The investigating officers yesterday arrested actress Kajal Agarwal's manager Puttkar Ronson Joseph in connection with the racket.
Since July 19, the SIT has questioned noted filmmaker Puri Jagannadh, cinematographer Shyam K Naidu and character actor P Subba Raju, besides Tarun Kumar.
The names of some Tollywood personalities came out during the interrogation of those arrested so far.
The SIT was quizzing them to find out if they had any links with the racket as consumers or suppliers or with those arrested, excise officials earlier said.
Excise Enforcement Director Akun Sabharwal yesterday said a total of 19 persons were arrested, seven cases booked and 27 questioned in connection with the racket. He said some more persons may be arrested, based on evidence.
Mr Sabharwal also said that the Prohibition and Excise department is within the purview of its powers to investigate any offence under the NDPS Act.
"We are legally empowered to investigate these cases.
The department is following all provisions of law, including the Supreme Court's guidelines, as far as interviewing and questioning is concerned of the summoned persons. We are within the parameters of law," he earlier said.
He said it has come to light that people are judging the investigation process without having full knowledge of it.
"I think it is incorrect on anybody's part to arrive at a judgement... We are following all legal provisions...it is incorrect on the part of some to pass a judgement on the investigation," he said.
"The accusations that the officers are not doing their duties properly also amount to defamation," Mr Sabharwal said.
He also said that no samples are collected without a written consent of the person. Women officers are already a part of the probe team, he added.
Actor Charmee Kaur, who is among those summoned, yesterday moved the Hyderabad High Court, saying she should be questioned in the presence of women officers and her lawyers.
The excise officials clarified that those being questioned were from different sections of the society, and denied that only film industry members were being targeted.
As a part of the racket, high-end drugs such as lysergic acid diethylamide (LSD) and methylenedioxymethamphetamine (MDMA) were being supplied.
The investigators suspect that film personalities, employees of MNCs, school and college students were among its clients.
The racketeers placed orders through the 'Darknet' (a restricted online network frequently used in illegal activities) and the drugs were delivered by couriers, including from overseas, the officials said.