WASHINGTON The U.S. Transportation Department (USDOT) on Wednesday said its comprehensive investigation into Southwest Airlines December meltdown is ongoing.

Southwest canceled 16,700 flights impacting more than 2 million passengers after its crew scheduling software failed to handle staffing changes.

The department first disclosed in January it was reviewing whether Southwest executives "engaged in unrealistic scheduling of flights which under federal law is considered an unfair and deceptive practice."

Southwest in a securities filing last week said it could face fines or penalties resulting from the USDOT or other governmental agencies.

(Reporting by David Shepardson; Editing by Mark Porter)