Email accounts of four top officials at the National Republican Congressional Committee (NRCC) were successfully hacked during the 2018 midterm elections.

The NRCC announced the hack on Tuesday through spokesman Ian Prior, and attributed it to “an unknown entity.”

“[U]pon learning about the intrusion, the NRCC immediately launched an internal investigation and notified the FBI, which is now investigating the matter,” said Prior.

The hack was first identified in April by MSSP, a security provider hired by the NRCC, but wasn’t disclosed because the committee’s investigation into the matter was still underway. House Republicans including Speaker Paul Ryan and Majority Leader Kevin McCarthy were unaware of the hack until Monday.

The NRCC hired CrowdStrike, the cybersecurity firm that assisted the Democratic Party in mitigating damage caused by Russian hackers during the 2016 election season, to investigate the intrusion.  The hacked emails themselves have not yet been made public.

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