Well, back in May, Mueller subpoenaed the Podesta brother's lobbying firm. The records he got would have detailed Manafort's activities in the Ukrainian election - which Hillary's State Department was very interested in.
Mercury had worked on the Ukraine lobbying project with the Podesta group, led by Anthony Podesta, brother of John Podesta, who led Hillary Clinton's presidential campaign.
http://www.chicagotribune.com/news/nationworld/politics/ct-mueller-subpoenas-lobbying-firms-russia-investigation-20170825-story.html