House Republicans are again battling with the Justice Department over information related to the Russia investigation, this time over documents the intelligence community said involves a top-secret source who has provided information to the CIA and FBI.

The mysterious source has also gathered information that was given to Special Counsel Robert Mueller as part of his investigation into Russian interference in the 2016 election, according to The Washington Post.

WaPo reported Justice Department and intelligence community officials issued a stark warning to the White House on May 2 against a request from House Intelligence Committee Chairman Devin Nunes. Nunes had submitted a subpoena to the Justice Department on April 24 for records related to the Russia probe.

Justice Department and intelligence community officials argued to White House Chief of Staff John Kelly that complying with the subpoena would reveal the identity of a top-secret source and would undermine protocol regarding intelligence sources, according to WaPo.

WaPo provided one small clue about the source: he or she is American.