The agency has also questioned members of screening committee including Secretaries and Joint Secretary in the Ministry, CBI sources said.
CBI sources said they will examine the Comptroller and Auditor General report tabled in Parliament today but will register any case only if some criminality is established during the probe.
They said the questioning of the Coal Secretaries, who also chair the screening committee, was done to understand the issues involved in the allocation of coal blocks during the period.
The agency has shortlisted nearly 15 companies which allegedly violated norms of allotment of coal blocks and questioning of their officials is going on, they said.
The agency has registered the Preliminary Enquiry (PE) , which is the first step of initiating the CBI probe, against unknown persons in connection with a complaint referred to it by the Central Vigilance Commission.
Allocation of coal blocks is done through an inter-ministerial inter-governmental body called the Screening Committee chaired by coal secretary.
It includes the secretaries of nine ministries having interests in coal, besides heads of various PSUs, and chief secretaries of coal-supplying states.