Special Counsel Robert Mueller has requested that Cambridge Analytica, a data firm that worked for President Donald Trump’s campaign, turn over documents as part of its investigation into Russian interference in the 2016 U.S. election, according to people familiar with the matter.

Mr. Mueller asked the firm in the fall to turn over the emails of any Cambridge Analytica employees who worked on the Trump campaign, in a sign that the special counsel is probing the Trump campaign’s data operation.

The special counsel's request, which the firm complied with, wasn’t previously known. The emails had earlier been turned over to the House Intelligence Committee, the people said, adding that both requests were voluntary.

On Thursday, Cambridge Analytica Chief Executive Alexander Nix interviewed via videoconference with the House Intelligence Committee, according to a person familiar with the matter.

Mr. Mueller’s request for employee emails was made before media outlets reported in October that Mr. Nix had contacted WikiLeaks co-founder Julian Assange during the 2016 campaign, according to a person familiar with the matter. The Sweden-based WikiLeaks last year published a trove of Hillary Clinton-related emails that U.S. intelligence agencies later determined had been stolen by Russian intelligence and given to the website.