The agency registered a preliminary inquiry after receiving a complaint from the Army Chief against unknown retired army officers and private persons, CBI sources said.
A preliminary inquiry is done before registering an FIR but during this initial probe the agency does not have powers to question, search or arrest a suspect.
The agency had yesterday received a complaint from Gen Singh giving details of the alleged bribery offer made to him by retired Lt General Tejinder Singh, who has denied the charge.
After getting legal opinion, the agency decided to proceed with a preliminary inquiry, the sources said. The CBI might convert the preliminary inquiry into an FIR if it finds prima-facie evidence of any alleged wrong-doing.
The Army Chief had claimed that an equipment lobbyist had offered him a bribe of Rs 14 crore, a matter he had reported to Defence Minister A K Antony.
The ministry had then recommended a CBI probe into the allegation made by the General.
In the initial communication to the CBI, the Army Chief had said that there was a bribe offer from Tejinder Singh following which the CBI had asked him to submit detailed description of the incident, list of possible witnesses and supporting documents on the issue.
After nearly 10 days, Singh had yesterday sent the complaint to the CBI in which he had given detailed description of the incident during which he was allegedly offered bribe.
Tejinder Singh has refuted the allegations leveled by the General and also filed a defamation case against the Army Chief and other officers.