The FBI is investigating a cyber breach that hit the House Republican campaign arm earlier this year, a spokesperson for the group said Tuesday.

The National Republican Congressional Committee (NRCC) was alerted to the breach in April, according to Politico, which first reported the intrusion. At that point, four committee aides had been surveilled for months, according to the news outlet.

Ian Prior, a vice president at the public relations firm hired by the NRCC to oversee its response to the breach, confirmed the intrusion in an email to The Hill.

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“The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity,” said Prior, a former Justice Department official. “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.”

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

None of the information accessed in the breach has been revealed publicly nor did anyone behind the intrusion threaten to expose the information, party officials told Politico.

Leading House Republicans, including Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (Wis.) and Majority Leader Kevin McCarthy Kevin Owen McCarthyMcCarthy's Democratic challenger to launch first TV ad highlighting Air Force service as single mother Trump asked Chamber of Commerce to reconsider Democratic endorsements: report The Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill MORE (Calif.), were not told about the episode until after Politico began asking the committee about it.

It’s not clear who was behind the breach or how long it lasted before committee officials were made aware. Politico reported that unidentified party officials believe a foreign agent may have been responsible.

The news of the hack comes after high-profile breaches at the Democratic National Committee during the 2016 presidential election raised concerns about cybersecurity in U.S. campaigns and elections.

The NRCC employs a full-time cybersecurity staffer and has procedures in place to safeguard certain information from potential intrusions.

Updated at 12:43 p.m.