The sources clarified that direct evidence of involvement of Railway Minister Pawan Kumar Bansal has not surfaced so far, but at least four officials posted in his office would be questioned very soon.
They, however, said Bansal's clarification would be sought in connection with the alleged bribery case.
The CBI is still analysing over 1,000 tapped phone conversations intercepted by it during the period of over two months as part of its surveillance on now suspended Railway Board Member (Staff) Mahesh Kumar and Bansal's nephew Vijay Singla, they said.
The complicity of the officials in the Rs 10 crore bribery scandal will be decided after sustained interrogation of the arrested accused which include Singla and Kumar, the sources claimed.
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Kumar, Singla and some other accused were giving conflicting replies and evasive in responses during their questioning, they said.
The probe agency might explore options such as polygraph tests to bring out truth from them, they said.
The sources said the money was allegedly a part payment of the Rs 10 crore promised for lateral shifting of Kumar from Member (Staff) to Member (Electrical), which is considered a coveted position in the Railway Board.