Special counsel Robert Mueller has asked data firm Cambridge Analytica to turn over documents related to campaign work for President Donald Trump.
The special counsel requested emails from any employees who worked on the Trump campaign as part of the ongoing investigation into election interference by Russia, reported the Wall Street Journal.
The firm voluntarily complied with Mueller's request and turned over emails which had previously been provided to the House Intelligence Committee, the newspaper reported.
CEO Alexander Nix was interviewed Thursday via videoconference by investigators on that House panel.
Mueller requested the emails before reports were published in October that Nix had contacted WikiLeaks co-founder Julian Assange during the 2016 campaign, the Journal reported.
WikiLeaks published thousands of emails stolen from the Democratic National Committee and Hillary Clinton campaign officials, which U.S. intelligence agencies believe was hacked by Russian intelligence operatives.
Top Trump donor Rebekah Mercer, a part owner of the firm with her father, asked Nix whether the data firm could better organize the stolen emails released by WikiLeaks.
Robert Mercer, the billionaire hedge fund manager and Trump megadonor reportedly has a $10 million stake in Cambridge Analytica and until recently funded Breitbart News, which is overseen by Steve Bannon.
Bannon introduced Nix to the campaign in May, and he took over as the Trump campaign's chief executive officer in August 2016, after now-indicted Paul Manafort was forced out over his ties to Russia.