Washington (CNN) Emails from top officials at the National Republican Congressional Committee were hacked during the 2018 midterm elections, Republican sources tell CNN, exposing the GOP's House campaign arm to an intrusion by an "unknown entity."

The hack, which was first reported by Politico , was discovered by a vendor in April after emails from four senior committee aides had been surveilled for months, a Republican official with knowledge of the intrusion tells CNN.

A source familiar with the investigation into the hack told CNN that the attackers would have been able to see all the emails being sent and received by the NRCC aides whose accounts were breached.

The attackers could have signed into those officials' accounts as if they were the officials themselves, the source said. To do this, the source said the attackers had obtained the passwords belonging to the officials. The source would not say how the attackers obtained the passwords.

The attackers did not have access to other NRCC systems as a result of this breach, the source added.

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