The U.S. Justice Department has started an investigation into whether top airlines have complied with contracts to carry mail cargo for the U.S. Postal Service, the three biggest U.S. carriers said. Delta Air Lines Inc and American Airlines Group Inc have each received a civil investigative demand from the U.S. regulator about the Postal Service mail they carry, spokesmen for the companies said. A similar inquiry by the Justice Department was disclosed by rival United Continental Holdings Inc, which said it was asked in October to provide documents and oral testimony about the scanning of international mail carried under its U.S. Postal Service contracts. The three airlines said they are fully cooperating with the investigation. Another airline, budget carrier Southwest Airlines Co, does not carry mail directly for the U.S. Postal Service and is not a party to the inquiry, a company spokesman said.