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CNN  — 

The US State Department recently detected a breach of its unclassified email system, according to a State Department spokesperson.

“The Department recently detected activity of concern in its unclassified email system, affecting less than 1% of employee inboxes,” the spokesperson said in a statement.

It’s not clear when the breach occurred or how long the system was vulnerable. The breach resulted in some employees’ personal information being “exposed.”

There was no detection of any activity in State’s classified system, the spokesperson said.

“This is an ongoing investigation and we are working with partner agencies, as well as the private-sector service provider, to conduct a full assessment. We will reach out to any additional impacted employees as needed,” the spokesperson said.

Politico was first to report this story.

The State Department’s email system is a high-profile target for hackers.

CNN reported in 2015 that Russian hackers had broken into the State Department’s email system in the “worst ever” cyber attack against a federal agency.

In 2014, the State Department went so far as to shut down its email system for a weekend to try to improve security and block intruders.