ACB chief M K Meena confirmed that the senior bureaucrat was questioned, but did not disclose any details.
According to ACB sources, the queries mostly revolved around action taken by Kumar after the anti-corruption bureau submitted a report in 2013 in connection with the alleged irregularities, ACB sources said.
In 2013, Kumar, who is now Principal Secretary to Chief Minister Arvind Kejriwal, was the transport commissioner.
When asked about the nature of queries put forth to Kumar by the ACB, the Delhi Government did not react.
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The sources claimed that the probe was stalled due to inability to get permission to question the officials.
On September 3, the ACB filed a charge sheet in a court in connection with the scam, in which several persons were arrested in 2013 for allegedly siphoning off crores of rupees in conducting fitness tests of commercial vehicles in the national capital in connivance with the transport department.