The email accounts of four senior aides at the National Republican Congressional Committee were surveilled for several months, the party officials said. The intrusion was detected in April by an NRCC vendor, who alerted the committee and its cybersecurity contractor. An internal investigation was initiated and the FBI was alerted to the attack, said the officials, who requested anonymity to discuss the incident.

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.