(Reuters) – Southwest Airlines (NYSE:LUV) said on Monday the U.S. transportation regulator may impose a fine related to the wide-scale operational disruptions the carrier faced between December 21 and 29 last year.

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

A probe by the Department of Transportation found the airline failed to provide “adequate customer service assistance, prompt flight status notifications, and proper and prompt refunds.”

The department notified Southwest on Oct. 27 that the assessment of a civil penalty for its December flight upheaval was warranted, the carrier revealed in a securities filing on Monday.

The airline intends to engage with the department to resolve the matter, which may involve a civil penalty.