The National Republican Congressional Committee says four of its senior aides were hacked earlier this year.

Committee spokesman Ian Prior told Fox News that the organization "was the victim of a cyber intrusion by an unknown entity."

"The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity," Prior said. "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."

According to Politico , the hack exposed thousands of emails from accounts of four senior aides at the committee containing sensitive information. Committee officials said the email accounts of the aides had been surveilled for months when the hack was detected in April.

Donor information was not compromised in the hack and no information contained within the hacked email accounts has been revealed to the public.

"We don't want to get into details about what was taken because it's an ongoing investigation," a senior committee official told Politico. "Let's say they had access to four active accounts. I think you can draw from that."

The intrusion was uncovered by a security service provider that monitors the committee's network. The provider told the group's cybersecurity contractor, and the organization launched an internal investigation and informed the FBI. However, Politico reported that senior Republicans, including House Speaker Paul Ryan and Majority Whip Steve Scalise, were not aware of the hack until Monday.

During the midterm elections, the NRCC paid hundreds of thousands of dollars to two law firms to oversee the handling of the hacking.