In the massive breach of personal information at the Office of Personnel Management, it has been discovered that the Chinese had access to security clearance data for a year.
The discovery means that the hackers had a chance to gather extensive amounts of information over a long period of time,
The Washington Post reported.
"The longer you have to exfiltrate the data, the more you can take," Stewart Baker, a former National Security Agency general counsel, told the Post. "If you've got a year to map the network, to look at the file structures, to consult with experts and then go in and pack up stuff, you're not going to miss the most valuable files."
Agency officials said that the compromise of the system dates back to last summer.
The database holds detailed information about 4 million current, former and prospective federal employees. The hack dates back to December but was only discovered in April.
OPM officials are still trying to determine how much information was stolen and who was affected.
"This is some of the most sensitive nonclassified information I could imagine the Chinese getting access to," Baker told the Post.
The Obama administration has not named who it thinks perpetrated the intrusions but officials are privately pointing fingers at the Chinese government.
Senior U.S. officials told the Post that the Chinese have in the last 12 to 18 months built vast databases of Americans' personal information to use for counterintelligence, using data from a variety of different hacks, including the breach of health insurance company Anthem.
The OPM has come under fire for the way it has handled numerous network hacks over the last year and a half, but under the leadership of the agency's director, Katherine Archuleta, the agency has been able to identify vulnerabilities and has tried to begin addressing them by modernizing the network.
The Post said, however, that government bureaucracy prevented the department from gaining access to security tools quickly, according to officials.
OPM officials defended the work that had been done to try to secure the systems.
"The only reason we even found these breaches was because of Director Archuleta's strategic plan, which she put in place within her first 100 days," spokesman Samuel Schumach told the Post.