The email accounts of four senior aides of the National Republican Congressional Committee were monitored for months during a 2018 hack, exposing thousands of messages to a yet-unidentified culprit, according to a Tuesday report from Politico. The committee first learned of the breach in April 2018 after it was spotted by an NRCC security provider—but even senior House Republicans were not informed of the attack. One of Politico’s sources said the committee didn’t tell the public—or even their own party—about the attack because they feared compromising the investigation. The hacker’s identity remains unknown, although the committee suspects that it was a foreign attack. Party officials told Politico that none of the stolen information has been made public, and that they have not received any threats regarding its exposure.