The Accountant General (Audit), Central Revenues, has been asked whether rules were flouted for procurement of nearly 400 water tankers, official sources said.
ACB chief Mukesh Kumar Meena confirmed the development, but didn't divulge further details.
The anti-graft body has been probing the scam that dates back to the tenure of former chief minister Sheila Dikshit.
Sacked Delhi minister Kapil Mishra had been questioned by the agency after he approached the anti-graft body with some documents related to the "scam".
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Kejriwal's private secretary Bibhav Kumar too was questioned last week by the ACB in the case.
In August last year, Dikshit was "examined" by ACB officials for her alleged involvement in the tanker scam.
The Kejriwal-led AAP government had in June 2015 constituted a fact-finding committee to go into the alleged irregularities in procuring 385 stainless steel water tankers by the Delhi Jal Board.
In June 2016, the government had sent the report of the fact-finding committee to the then Lt Governor Najeeb Jung, following which an FIR was registered in the case.