Computers containing data relating to national security and VVIPs like Prime Minister Narendra Modi were compromised in early September after a security breach at the NIC (National Informatics Centre), Delhi Police sources have said.
The computers affected also stored data relating to National Security Advisor Ajit Doval, Indian citizens and senior government functionaries.
The NIC sets up and maintains information and communication technology and security for the government.
Delhi Police's Special Cell filed a case based on a NIC employee's complaint immediately after the attack.
According to sources the attack began with the receipt of an e-mail. When the employee clicked on attachments in that mail, data stored on that machine became compromised and other computers were affected.
The email was traced to an IT company in Bengaluru.
However, according to news agency ANI the email was sent from the United States via a proxy server to the Bengaluru-based firm.
Further investigations are ongoing, the sources said.
The breach comes amid allegations a Chinese firm - Zhenhua Data Information - is conducting covert surveillance on thousands of Indians, including the Prime Minister, the President, the Vice President and the Army Chief.
Sources have said the government has constituted an expert committee (under the National Cyber Security Coordinator) to study these allegations. The committee is to submit a report inside 30 days.
The issue has also been raised by the Foreign Ministry with Sun Weidong, the Chinese Ambassador to India. The Chinese government has said Zhenhua is a privately-owned company with no links to Beijing.
The row over alleged snooping and surveillance by Chinese firms comes amid a serious military stand-off between the two countries along the LAC (Line of Actual Control) in Ladakh.
Tensions have spiked since a violent confrontation in the Galwan region in June, in which 20 Indian soldiers were killed in the line of duty.
Multiple talks at military, diplomatic and ministerial levels have, so far, failed to resolve the issue. The foreign ministers met in Moscow and hammered out a five-point plan to defuse the tension.
According to news agency PTI, in June, days after the Galwan incident, the Telecom Department told state-run firms MSNL and BSNL to avoid using Chinese equipment in upgrading their 4G facilities.
With input from ANI, PTI