Data stolen from U.S. government computers by suspected Chinese hackers included security clearance information and background checks dating back three decades, U.S. officials said Friday, underlining the scope of one of the largest known cyberattacks on federal networks.

The breach of computer systems of the Office of Personnel Management (OPM) was disclosed Thursday by the Obama administration, which said records of up to 4 million current and former federal employees may have been compromised.

Accusations by U.S. government sources of a Chinese role in the cyberattack, including possible state sponsorship, could further strain ties between Washington and Beijing. Tensions are already heightened over Chinese assertiveness in pursuit of territorial claims in the South China Sea.