The CBI has started collecting documents regarding companies who were allegedly allocated coal blocks during 2006-09 and has approached coal ministry regarding it.
CBI sources said it is verifying the contents of reference received from the Central Vigilance Commission on alleged irregularities in coal block allocation and utilisation.
The agency has asked the concerned officials in the ministry to provide documents related to the allotment of coal blocks to beneficiaries.
The agency registered a preliminary enquiry on Friday, which is the first step to start a probe, against unknown persons.
Senior CBI officials had said that probe would primarily focus on the alleged irregularities in utilisation of coal blocks by the allottee companies between 2006 and 2009.
The Commission had forwarded the complaint of BJP leaders Prakash Javadekar and Hansraj Ahir who had alleged that first-come-first-serve basis was adopted by the government to benefit some private companies.
The complaint also alleges that there was a delay of nearly two years in auctioning process and claims that the ministry had done so to benefit some private players.
If CBI finds any incriminating evidence during the enquiry, it would register a regular case.