The State Department revealed that its unclassified email system was hit by a data breach, impacting fewer than 1 percent of employees' inboxes.

The State Department disclosed in a Sept. 7 alert that there was an “activity of concern … affecting less than 1% of employee inboxes,” according to Politico, who was the first to report the incident Monday.

“We have determined that certain employees’ personally identifiable information (PII) may have been exposed,” the alert said. “We have notified those employees.”

However, not all email systems were impacted by the breach. For example, the classified email system labeled “Sensitive But Unclassified” was not affected.

"Like any large organization with a global presence, we are a constant target for cyber attacks ... We have not detected activity of concern in the Department’s classified email system," a State Department official told the Washington Examiner.

The State Department is conducting an investigation into the breach, though it is unclear if the department has identified who was responsible for the breach.

"We are working with the interagency, as well as the private sector service provider, to conduct a full assessment," the State Department official said. "The Department is always actively engaged in identifying cyber security threats and protecting its networks. This is an ongoing investigation. We have no additional information to share at this time."