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WASHINGTON — The emails of top officials at the campaign arm for House Republicans were stolen during the 2018 midterm election cycle, a spokesman confirmed to NBC News on Tuesday.

Ian Prior, a spokesman for the National Republican Campaign Committee, said that the group had launched an internal probe and flagged the attack to the FBI.

"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."

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"To protect the integrity of that investigation, the NRCC will offer no further comment on the incident," he added.

The hack, which was first reported by Politico, involved the email accounts of four senior aides at the NRCC, said two sources with knowledge of the situation. Those accounts were surveilled for months, said the sources, who added that no donor information was compromised.

The intrusion was first noticed in April by an NRCC vendor, which then notified the committee and its cybersecurity contractor. The hack is believed to have been perpetrated by a foreign agent, due to the nature of the attack.

NBC News has also confirmed that top Republican party leaders — including Speaker Paul Ryan, Majority Leader Kevin McCarthy and Majority Whip Steve Scalise — were only made aware of the hack Monday.

The information stolen in the hack has not appeared in public. The NRCC has hired and paid hundreds of thousands of dollars to both Mercury Public Affairs and Washington-based law firm Covington and Burling to respond to the hack, Politico said.

The news comes as Congress and special counsel Robert Mueller continue to investigate email hacks and interference by Russia in the 2016 presidential election. The campaign arm for the Democrats, Democratic Congressional Campaign Committee (DCCC), and the Democratic National Committee, were among the groups hacked during the campaign cycle.

NBC News’ Pete Williams and Ken Dilanian reported in October that the Department of Homeland Security was working to identify who or what was behind an increasing number of attempted attacks on U.S. election databases in the runup to the midterms.

House Republicans ultimately lost around 40 seats in last month's election, losing their majority to Democrats.