A source familiar with the situation said on Thursday the U.S. Department of Justice has significant concerns about Aetna Incorporated's proposed acquisition of health insurer Humana Incoporated. In response, shares of Humana fell more than 11 percent. According to Reuters, Aetna's purchase of Humana would combine two of the largest providers of Medicare Advantage plans for elderly people. Investors have long been concerned the deal might pose a competition issue for antitrust regulators. The Justice Department is also reviewing Anthem Incorporated's proposed purchase of Cigna Corporation. Investors are worried it will fail because of its impact on competition in the employer insurance market.