Hundreds, if not thousands, of workers have missed out on overtime and holiday pay in recent weeks. In Cleveland, city administrators set up a "war room" to deal with the paycheck backlog.

The Biden administration, backed by U.S. allies, is publicly calling out Beijing for an attack this year on Microsoft's Exchange email server software that targeted computers worldwide.

When COVID hit, a Chinese firm offered to set up testing labs in the U.S., which could have given it access to DNA data. The U.S. says this is part of China's effort to collect mass data on Americans.

In its most detailed comment so far, the U.S. government says the breach of government and private company computer networks "will require a sustained and dedicated effort to remediate."

The California-based company is often called by governments and companies around the world to protect against hacks and respond to breaches. FireEye acknowledged that some valuable tools were stolen.