The emails of four senior aides were being monitored for months, an intrusion that was detected in April by an NRCC vendor, who told the committee and its cybersecurity contractor.
Politico broke the news this afternoon, and a firm the NRCC brought on to help with the response said in a statement, "The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity".
Unnamed party officials told Politico that they believe a foreign agent was behind the attack.
The NRCC said it notified the FBI, and launched an internal investigation.
An internal investigation and FBI investigation are ongoing.
The NRCC and the Department of Justice did not immediately respond to HuffPost's requests for comment.
Google later confirmed in September that the personal Gmail accounts of multiple senators and staffers had recently been targeted by foreign hackers, though it did not specify the cyberspies' nationality nor the party affiliations of the targets.
Republicans used hacked information from the Democratic National Committee to their advantage in 2016. But hackers often launch sweeping spear-phishing campaigns to gain access to a variety accounts - with no political motivation. "I think you can draw from that".
Prior, who works for Mercury Public Affairs, said the NRCC will not be commenting further on the incident.
Cybersecurity became a huge issue for campaigns after Russians hacked the Democratic National Committee and Hillary Clinton campaign chief John Podesta during that contest. It is unclear to what extent, if at all, the NRCC hack could have contributed to that.
NRCC officials said that while thousands of emails were compromised, they insisted that no donor information was exposed, and the organization was never threatened with the information being leaked to the public.