In an affidavit filed before the apex court, the agency said that it is taking up the probe against each and every company which has been allocated coal block since 1993 and "in particular during the period 2006 to 2008."
"CBI learnt that during the period 1993 onwards there were irregularities in the allocation of coal blocks and awarding contracts for development of mines and formation of joint ventures by public sector undertakings under the Government dispensation category.
"The concerned authorities did not follow the rules and procedures for awarding the mines development contract for the coal blocks allocated under Government dispensation category by Ministry of Coal," the affidavit said.
The agency said that a special team comprising investigators specifically selected by the CBI Director has been formed and they are scrutinising 700 files running into approximately 1.60 lakh pages.
"The system adopted by the concerned authorities in formation of joint ventures was not transparent and that due diligence was not observed during the process.
"Some public servants had vested interests in the award of contract for the development of mines or joint ventures with private parties. It was also suspected that large number of private companies had unduly benefited in the process," the agency said.