ATLANTA – Delta Air Lines will begin diversity training for all of its flight crews after a series of publicized incidents in which passengers, including Muslims and African Americans, said they were discriminated against or treated uncivilly on its flights.

Those incidents include a flight attendant appearing reluctant when a Black passenger said she was a doctor and offered to help a sick passenger, as well as a number of incidents involving Muslim or Arab speaking passengers who claimed they were kicked off flights out of suspicion.

U.S. Department of Transportation issued new guidelines for airline personnel outlining their legal obligation not to discriminate against passengers based on race, sex, religion or national origin.

The Atlanta-based airline is last among the four largest U.S. carriers to require such training, which it made mandatory for executives last year and will start for 23,000 flight attendants in the second quarter. Delta’s classes will use “real and relevant scenarios” and discuss unconscious bias and so-called microaggressions that will “go well beyond the typical cross-cultural training.”

Tensions between passengers or between passengers and crew have been on the rise. On Friday, the U.S. Department of Transportation issued new guidelines for airline personnel on nondiscrimination, including sample scenarios outlining their legal obligation not to discriminate based on race, sex, religion or national origin. It also issued a document for passengers, explaining their rights.

Incidents of reported discrimination rose 37 percent in the first 10 months of 2016 to 74, according to the transportation agency.

The U.S. presidential campaign, in which Trump proposed banning Muslims from the U.S., and recent terrorist incidents have intensified the issue.

The four Transportation Department scenarios included a headscarf-wearing woman, two men speaking Arabic and a Sikh. The department advised flight crew to react to passengers based on their behavior, rather than their appearance.

American Airlines Group, the world’s largest carrier, has a “value of respect” training that’s required for all flight attendants with a refresher every two years. Southwest Airlines gives mandatory anti-discrimination and anti-harassment courses to new hires, and optional diversity training thereafter.

At United Airlines, all customer-facing employees, including its 25,000 flight attendants, undergo recurrent diversity training that includes lessons in cultural awareness.