Hackers broke into U.S. government computers owned by the Office of Personnel Management (OPM) and possibly compromised the personal data of 4 million current and former federal employees. OPM on Thursday said that it discovered the attack affecting its information technology (IT) systems in April 2015. It was not specified what kind of information was accessed.

Since the incident was identified, OPM has partnered with the U.S. Department of Homeland Security’s U.S. Computer Emergency Readiness Team (US-CERT), and the Federal Bureau of Investigation to determine the impact to Federal personnel. OPM also implemented additional security measures to protect the sensitive information it manages.

Beginning June 8 and continuing through June 19, OPM will be sending notifications to approximately 4 million individuals whose Personally Identifiable Information was potentially compromised in this incident. The email will come from opmcio@csid.com and it will contain information regarding credit monitoring and identity theft protection services being provided to those Federal employees impacted by the data breach.

In order to mitigate the risk of fraud and identity theft, OPM is offering affected individuals credit monitoring services and identity theft insurance with CSID, a company that specializes in identity theft protection and fraud resolution. This 18-month membership includes credit report access, credit monitoring, identity theft insurance, and recovery services and is available immediately at no cost to affected individuals identified by OPM.

Cyber investigators linked the breach to earlier thefts of healthcare records from Anthem, the second largest U.S. health insurer, and Premera Blue Cross, a healthcare services provider.

In addition, Reuters repost that a "foreign entity or government" was believed to be behind the cyber attack, citing an unnamed U.S. law enforcement source.

But Beijing denied any involvement.

"Cyber-attacks are generally anonymous and conducted across borders and their origins are hard to trace," foreign ministry spokesman Hong Lei said at a regular briefing.

"Not to carry out a deep investigation and keep using words such as 'possible' is irresponsible and unscientific," he added.