At a congressional hearing where lawmakers voiced outrage over the hacking incidents, the head of the Office of Personnel Management warned that more bad news may be coming.
OPM chief Katherine Archuleta did not mention China -- which has been widely blamed for the incidents -- but told the hearing that "these adversaries are sophisticated, well funded and focused. These attacks will not stop. If anything, they will increase."
Archuleta said that without recently implemented security measures "we would have never known that malicious activity had previously existed on the network, and would not have been able to share that information."
"As our detection methods continue to improve, more events will come to light," he said.
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The agency indicated in early June that some four million current or former government employees were hit. Later reports said the breach may have affected all federal employees as well as contractors and retirees, and could include sensitive data from background investigations.
Archuleta said that "there is a high degree of confidence that systems related to background investigations ... May have been exfiltrated."
As the hearing opened, lawmakers berated the government for failing to heed repeated warnings about weaknesses in computer networks over the past few years.
Chaffetz said vulnerabilities were highlighted in several internal reports and that failure to act was "akin to leaving all the doors and windows of your house open."
Chaffetz said inspector general reports dating back to 2007 identified these problems.
"For any agency to consciously disregard its data security for so long is grossly negligent," he said.
Representative William Hurd said the incident is "another reminder of the undeniable fact that America is under constant attack... Our enemies are attempting to rob our people on a daily basis."