The anti-graft body had summoned Bibhav Kumar last week and he joined the probe on Wednesday.
"He arrived at 11.30 am for the questioning at the ACB office. His questioning is currently underway," said a senior officer.
Kumar had been asked to bring some files and they are being examined.
The ACB had recorded sacked Delhi minister Kapil Mishra's detailed statement last week and he will be questioned on certain points on Thursday.
Mishra had alleged that Kejriwal had influenced the probe into the tanker scam that dates back to Sheila Dikshit's tenure as chief minister.
The sacked minister also claimed that Kejriwal's aide had asked him to delay sending of a few reports to the then Lieutenant Governor Najeeb Jung in connection with the alleged scam, sources said.
Mishra had alleged that the investigation into the tanker scam was repeatedly delayed and influenced by Kejriwal and "his two men".
Last year in August, Dikshit was "examined" by the ACB officials in connection with her alleged involvement in the tanker scam besides being handed over a set of 18 written questions.
Till some time back, Mishra had been accusing the BJP of shielding Dikshit in the case but a couple of weeks back, he alleged that the AAP government tried to protect her.
The Kejriwal-led AAP government had in June 2015 constituted a fact-finding committee to go into alleged irregularities in procuring some 385 stainless steel water tankers by the DJB.
The Delhi government in June 2016 sent a report of the fact-finding committee to Jung following which an FIR was registered in the case.