Humana Inc., the giant Louisville, Ky.,-based health insurer, faces multiple federal investigations into allegations that it overbilled the government for treating elderly patients enrolled in its Medicare Advantage plans, the Center for Public Integrity reported Friday. The center, citing court records, said the status of the investigations is not clear, but they apparently involve several branches of the Justice Department. Among them are probes by the U.S. Attorney's branch office in West Palm Beach, Fla., and the criminal division of the Justice Department in Washington into allegations of overbilling and fraud against Humana, which insures more than 2 million people through the Medicare Advantage plans.