Delta Air Lines is facing a $2 million federal fine for violating rules on serving passengers with disabilities, officials at the U.S. Department of Transportation said Thursday.

The fine is the largest ever assessed on an airline for a non-safety matter.

It comes after federal investigators say that Delta failed to properly assist individuals with disabilities in getting on and off aircraft on many occasions.

Advertisement - Continue Reading Below

In addition, the airline frequently did not respond to written disability complaints from passengers, as is required under federal rules, and failed to report all complaints to the Department of Transportation.

“Ensuring that passengers with disabilities receive fair treatment when they fly is a priority for the Department of Transportation,” said Transportation Secretary Ray LaHood. “We take our aviation disability rules seriously and will continue to enforce them vigorously.”

Up to $1.25 million of the fine may be used by the airline to improve service to passengers with disabilities.