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WASHINGTON, Dec.4 (Xinhua) -- The U.S. National Republican Congressional Committee (NRCC) was hacked during the 2018 election cycle, exposing sensitive emails, a spokesman for the campaign arm of House Republicans said on Tuesday.

The NRCC can confirm that it was the victim of "a cyberintrusion by an unknown entity," Ian Prior, a spokesman for the committee, said in a statement.

"The cybersecurity of the committee's data is paramount, and upon learning of the intrusion, the NRCC immediately launched an internal investigation and notified the FBI, which is now investigating the matter," Prior said.

Local media reports said the email accounts of four senior NRCC aides were "surveilled for several months" and exposed "thousands of sensitive emails to an outside intruder."

The breach was detected in April, but the hackers penetrated the system a few months before that, according to a NPR report.

The FBI generally does not confirm or deny the existence of investigations.