BEIJING — China accused the United States of making "groundless accusations" and being "irresponsible" Friday in blaming Chinese hackers for a vast data breach that could be the biggest cyberattack in U.S. history.
Four million federal workers may have had their personal information compromised in the attack, which officials said could affect every agency of the U.S. government.
U.S. officials and lawmakers identified the likely culprit as China, which has been suspected of involvement in previous government hacks.
Sen. Susan Collins (R-Maine), a member of the Senate Intelligence Committee, said the hack was "extremely sophisticated," and "that points to a nation state" as the responsible party, likely China.
"We're still evaluating how serious the breach is, but if it does involve the compromising of the personal records of four million Americans, I certainly think that a strong response is warranted," she said.
However, China's foreign ministry spokesman Hong Lei told NBC News that it was very hard to prove who was responsible for cyber attacks.
"Without the thorough investigation, you jump to a conclusion so quickly. We think it is not scientific and is irresponsible."