The Department of Justice has significant concerns about a proposed merger between two major health insurers, Aetna and Humana, according to reports.

Humana shares fell 11 percent at news that Justice officials are scheduled to meet Friday with the two companies to discuss concerns about the deal, which would combine two of the largest providers of Medicare Advantage plans to seniors and further consolidate the insurance industry in a trend that’s been troubling to antitrust regulators.