Emails of four top National Republican Congressional Committee aides were hacked, according to a Politico report published Tuesday.

For several months, the NRCC email accounts of these four top aides was surveilled. The data breach was discovered by an NRCC vendor in April who immediately alerted the cybersecurity team. The FBI was also informed of the hack, reports Politico.

“We don’t want to get into details about what was taken because it’s an ongoing investigation,” said a senior party official. “Let’s say they had access to four active accounts. I think you can draw from that.”

The hack became a major source of consternation within the committee as the midterm election unfolded. The NRCC brought on the prominent Washington law firm Covington and Burling as well as Mercury Public Affairs to oversee the response to the hack. The NRCC paid the two firms hundreds of thousands of dollars to help respond to the intrusion. The committee’s chief legal counsel, Chris Winkelman, devoted hours of his time to dealing with matter.

Party officials would not say when the hack began or who was behind it, although they privately believe it was a foreign agent due to the nature of the attack.

Donor information was not compromised during the intrusion, the party officials said.

“The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity. 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,” said Ian Prior, a vice president at Mercury.

Prior, a former Justice Department official and NRCC operative, has been working with the committee to deal with the matter.

“To protect the integrity of that investigation, the NRCC will offer no further comment on the incident,” Prior added.

So far, none of the information contained in the emails has been made public.

Top Republicans were not aware of the hack until Politico contacted their offices for comment. NRCC officials said they remained tight-lipped so as not to compromise the investigation into the hack. Politico ctd:

However, senior House Republicans — including Speaker Paul Ryan (R-Wis.), House Majority Leader Kevin McCarthy (R-Calif.) and Majority Whip Steve Scalise (R-La.) — were not informed of the hack until POLITICO contacted the NRCC on Monday with questions about the episode. Rank-and-file House Republicans were not told, either.

Rep. Steve Stivers (R-Ohio), who served as NRCC chairman this past election cycle, did not respond to repeated requests for comment.

Committee officials said they decided to withhold the information because they were intent on conducting their own investigation, and feared that revealing the hack would compromise efforts to find the culprit.