The government is fining Delta Air Lines $2 million for violating rules to protect disabled passengers.

It's the largest fine ever levied against an airline by the U.S. Department of Transportation, aside from penalties for safety violations.

The DOT said an investigation of Delta covering 2007 and 2008  revealed "many violations" of the requirement to assist disabled passengers on and off planes. Delta also often did not adequately respond to passengers' disability complaints and did not report each complaint in reports to the DOT, the agency said.

"We take the responsibility of serving customers with disabilities seriously," Delta said in a written statement.

According to a consent order, Delta must pay $750,000 in the next month but can offset the remaining $1.25 million through service improvements for disabled passengers, including adding a wheelchair tracking system at major hubs and surveys, and improving its website and complaint audits.

The Atlanta-based airline said it has already made investments since 2008 and will continue to work with the DOT.

In 2009, DOT data show that Delta reported roughly 1,300 complaints for failing to provide assistance to passengers requiring wheelchairs, out of 2,377 total disability-related complaints reported by Delta that year.

The Atlanta-based Shepherd Center, a well-known hospital for people with spinal cord or brain injuries, rose to the airline's defense. Chairman James Shepherd said in a written statement that Delta has a complaint rate of less than one-tenth of 1 percent among passengers with disabilities and the airline has placed an "incredible focus" on continuously improving its services.

Delta has donated to the Shepherd Center, and the hospital's director of advocacy sits on Delta's customer advisory board on disabilities.