Delhi Police Commissioner B S Bassi said the documents were photocopied and leaked in return for money and the investigators will soon apprehend the recipients of the papers.
The Police Commissioner said the confidential documents were leaked to certain independent consultants and energy companies some of whom are being interrogated.
Terming it as a serious case, Petroleum Minister Dharmendra Pradhan said strong action would be taken against the the employees of his ministry who allegedly leaked the documents to private companies.
Asked whether any corporate lobbyist could be involved, the Petroleum Minister said it was up to the police to investigate.
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"The government was conscious. Agencies are investigating. They will come out with full facts," he said.
The Poice Commissioner said one of the persons used to enter the Shastri Bhavan using forged identity cards to gain access to the ministry documents. A number of persons, Bassi said, are being questioned.
The Reliance Industries said it has launched a "robust internal probe" into detention of one of its employees by Delhi Police in connection with alleged official document theft in the Oil ministry.
The company said no information of commercial consequence to it resides in the ministry with which it is in arbitration in several cases.