The U.S. government has discovered that hackers have broken into government computers, risking personal data of over 4 million federal employees, current and former. U.S. officials said Thursday that they suspect the hackers to come from China.

The current security breach is described by the U.S. Office of Personnel Management as one of the largest information breaches ever involving government workers.

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OPM, the U.S. agency handling employee records and security clearances, is reportedly the federal government's equivalent to the Human Resources department of a private company. OPM still has not pinpointed the information taken by the hackers, but the agency thinks the info can be used in "spear-phishing" attacks. These attacks involve fake emails making targets think they are legitimate requests, according to Consumer Affairs.

The detection of new malicious activity in information systems was detected by OPM in April, and as May began, the Department of Homeland Security said date of the agency had been compromised, involving the estimated 4 million workers.

A "foreign entity or government" is the culprit behind the latest breack, according to a U.S. law enforcement, adding that authorities are investigating the possibility that the hackers could have come from China, according to Reuters.

Meanwhile, Spokesman Zhu Haiquan for the Chinese Embassy denied any involvement from China. He told NBC News that "Chinese laws prohibit cyber-crimes of all forms" and that "Jumping to conclusions and making hypothetical accusations is not responsible and counterproductive."

Ironically, this week is China's National Cyber Security Awareness Week, reported NBC News. The FBI had already started probes in an aim to indict those responsible for the breach.

The Chinese hackers were initially blamed since a large breach on OPM's computer the previous year. U.S. officials said the hackers targeted files of thousands of employees who had acquired for top-secret security clearances, in a New York Times report last July.

"The FBI is working with our interagency partners to investigate this matter," the bureau said via a statement. "We take all potential threats to public and private sector systems seriously, and will continue to investigate and hold accountable those who pose a threat in cyberspace."

OPM networks has had added security precautions since the hacks. It added that the 4 million employees would be notified immediately and that they would provide credit monitoring as well as identity theft services to those involved.

"The last few months have seen a series of massive data breaches that have affected millions of Americans," said U.S. Representative Adam Schiff in a statement.

He called the latest hack "among the most shocking because Americans may expect that federal computer networks are maintained with state-of-the-art defenses."

TagsUS cyber attack, Cyber Attack, US federal workers hacked, security breach, Chinese hackers