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(CNN) —  

Hackers may have stolen applications for security clearances from government employees, including those in the military and intelligence communities, from U.S. government computers, administration officials told CNN Friday.

The disclosure of a second breach to U.S. government computers comes one week after officials said up to four million federal employees’ personal information was hacked in an attack blamed on the Chinese.

The breach that was revealed on Friday stole data files that included applications for clearances up to “top secret,” one of the highest clearance levels, the sources said. Levels above top secret are generally held by people given special clearance to access information for particular programs, such as covert operations or technical data for sensitive national security projects.

Clearances higher than top secret may not have been compromised, though some employees with lower-level access may have unintentionally had some of their information placed at risk as well.

The U.S. government did not acknowledge the second hack at the time of the initial announcement, reported by CNN last week. Four million federal employees’ personal information is believed to be at risk in an attack at the Office of Personnel Management that U.S. officials blamed on Chinese hackers.

“During the investigation into the cyber-intrusion of OPM that compromised personnel records of current and former federal employees announced last week, investigators became aware of an additional intrusion affecting different OPM systems and data,” a senior administration official told CNN Friday.

One official said this second hack could post serious damage to national security, with another senior U.S. official calling it a “very big breach.”