U.S. officials said Thursday that hackers in China launched a massive cyber attack on the federal agency responsible for collecting background information and issuing security clearances for millions of government employees.
The Office of Personnel Management said that as many as 4 million current and former federal employees may have been affected. It said that number could go higher as the investigation continued.
Law enforcement officials told various media outlets that they thought Chinese hackers were behind the attack.
A Chinese Embassy spokesman in Washington, Zhu Haiquan, said China outlaws cyber warfare, adding that "jumping to conclusions and making hypothetical accusations is not responsible and counterproductive."
The top Democrat on the House Intelligence Committee, California's Adam Schiff, said this attack was most shocking "because Americans may expect that federal computer networks are maintained with state-of-the-art defenses."
OPM said it detected the security breach in April, before it made what it called an "aggressive effort" to implement tougher controls. It said the FBI and Department of Homeland Security were investigating to determine the full extent of the damage.
The FBI said it takes all threats to public and private sector cyber systems seriously and would hold those who make such threats accountable.
OPM said it would notify all current and former federal employees whose information might have been stolen. The agency will offer those workers access to credit reports and monitoring and identity theft recovery services at no cost.
The OPM cyber attack may be the biggest, but is not the first time hackers gained access to federal government computer systems.
Unclassified computers at the White House and State Department have been hit. Twitter and YouTube accounts of the U.S. central military command were also struck.
The Internal Revenue Service, which is responsible for tax collection, said last month that hackers stole information on 100,000 U.S. taxpayers.
Cyber warriors have also attacked such commercial giants as the Sony Pictures movie studio, Target and Home Depot stores, the eBay online auction site and JPMorgan Chase, a banking and financial services company.
Some of the attacks have been blamed on North Korea, Russia and China. Experts say China is desperate to get its hands on U.S. industrial and trade secrets. China angrily denies the accusations and says it has been targeted by U.S. hackers.
Schiff said late Thursday that it was "clear that a substantial improvement in our cyber databases and defenses is perilously overdue."