Special Counsel John Durham is challenging claims of attorney-client privilege over documents related to Perkins Coie and other groups that assisted Hillary Clinton’s 2016 campaign, records prosecutors say could otherwise be used as evidence in the upcoming trial of the firm’s former partner, Michael Sussmann.

Durham is asking the judge presiding over the case, U.S. District Judge Christopher “Casey” Cooper of the District of Columbia, to conduct a private review of the documents to determine if attorney-client privilege was appropriately invoked. Privilege logs and other evidence prosecutors have collected raise questions about the “validity, scope, and extent of the privilege assertions,” Jonathan Algor, a prosecutor assigned to Durham’s investigation wrote in a filing Wednesday.