"The CBI inquiry into the coal scam is not progressing as rapidly as expected. The probe is not being carried out in the manner and at the speed at which it should be by CBI.
"Moreover, the officials concerned in the PMO and coal ministry are not being questioned about their role in the scam," BJP spokesperson Prakash Javadekar said.
Singh had held the coal portfolio from 2006-09, when the allocations in question were made.
"The Prime Minister has further lost credibility. It is not our habit to demand his resignation, but there is a rationale behind it," Javadekar said.
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The main opposition also insisted that the alleged role of Minister of State for Coal, Dasari Narayan Rao, and Santosh Bagrodia, should be probed by CBI.
"CBI is not taking the probe forward. The money trail in the scam has still not become visible," Javadekar said.