"What makes this even more concerning is that, at the very moment these threats are escalating, this administration has been systematically hollowing out the very cyber expertise we rely on to defend the country, pushing out experienced professionals at the FBI and at Cybersecurity and Infrastructure Security Agency," he said (Sen Mark Warner).

The former cybersecurity official said U.S. adversaries are keenly aware that major reductions in the federal workforce provide a valuable opportunity to probe computer networks.

"They've got awareness at this point that the federal government is fairly hollowed out. If you're an adversary thinking about intelligence collection, this is certainly a time to take advantage of this opportunity," the former official said.