The US Justice Department has asked United Airlines for documents and oral testimony related to its contract to carry mail for the US Postal Service, the airline said in its annual regulatory filing.
The US agency's investigation regarded "delivery scan and other data purportedly required for payment for the carriage of mail," the filing said.
United, which received the civil inquiry last October 13, said it is responding to the Justice Department but cannot predict what action, if any, the regulator might take as a result of the investigation.
United spokeswoman Megan McCarthy added in a statement that the civil demand addressed international mail scanning requirements of its US Postal Service contracts.
"We believe the DOJ inquiry is industry-wide," McCarthy said. "We will continue to work with DOJ on its requests for information, and we are reviewing our mail scanning practices to ensure compliance."
Separately, United said in the filing that it is cooperating in a previously undisclosed investigation by the US Securities and Exchange Commission, which is related to another government probe surrounding the airline and the Port Authority of New York and New Jersey.
United's chief executive Jeff Smisek resigned in September in connection with an investigation that has addressed whether the company added flights to Columbia, South Carolina, when then-Port Authority Chairman David Samson had a home there.