CVS Health Corp. turned to Williams & Connolly trial lawyers this month as its $69 billion merger with Aetna Inc. faced an unexpected degree of scrutiny from a federal judge, who harangued the companies and the U.S. Justice Department for appearing to treat him as a “rubber stamp operation.”

The hiring of Williams & Connolly partner Enu Mainigi, along with two other lawyers at the firm, followed a hearing at which U.S. District Judge Richard Leon lambasted a Justice Department attorney for keeping him “in the dark” about CVS and Aetna combining their operations before the merger received any formal approval. The Justice Department in October approved the merger, with conditions.