U.S. authorities imposed a $4 million fine on German airline Lufthansa on Tuesday for denying boarding to 128 Jewish passengers on a flight in 2022, concluding they were discriminated against as part of the airline’s effort to handle the alleged misbehavior of some passengers.

The Department of Transportation said it was the largest civil rights fine it has ever imposed on an airline. Lufthansa has apologized for what happened and agreed to the fine, but it denied that any of its employees engaged in discrimination and disputed the authority of U.S. investigators to examine the case, according to an order resolving the incident.

The passengers were traveling from New York to Budapest via Frankfurt in May 2022 for an event honoring an Orthodox rabbi. Most of the passengers did not know each other and booked tickets individually or in small groups, according to investigators.

On the first leg of the trip, the crew alleged that some passengers had to be reminded not to congregate in aisles and galleys and had not complied with rules requiring masks that the German government imposed at the time. But rather than determine who had failed to follow the rules, the transportation department said Lufthansa decided to deny boarding on a connecting flight to the 128 passengers – many of whom officials said were men wearing distinctive Orthodox Jewish outfits.

Passengers interviewed by investigators said Lufthansa denied boarding “to everyone for the apparent misbehavior of a few, because they were openly and visibly Jewish.”

“No one should face discrimination when they travel, and today’s action sends a clear message to the airline industry that we are prepared to investigate and take action whenever passengers’ civil rights are violated,” Transportation Secretary Pete Buttigieg said in a statement.

Lufthansa said it had taken steps since the incident to deepen its relationship with Jewish communities, including partnering with the American Jewish Committee, an advocacy group.

“Through our ongoing collaboration, we have curated a first-of-its kind training program in the airline industry for our managers and employees to address antisemitism and discrimination,” the airline said in a statement. “Lufthansa is dedicated to being an ambassador of goodwill, tolerance, diversity, and acceptance.”

U.S. authorities concluded they had the power to investigate the incident because the flight originated in the United States. Airlines can deny boarding to passengers who present a security risk, but are not allowed to discriminate based on race or religion.

Investigators found that the captain on the first leg of the flight sent an alert to a security manager, who then placed a note on the passengers’ itineraries. Airline staff in Frankfurt decided to only board passengers without the notes on their tickets. About two dozen police were present at the gate for the connecting flight, passengers said at the time.

Videos from the airport posted at the time by travel blog DansDeals showed airline employees explaining the situation, including one who told a passenger that “it was Jewish people who were the mess, who made the problems.”

Lufthansa rebooked some of the passengers, while others made their own arrangements to reach their destination.

While it disputed authorities conclusion that it had discriminated, the airline acknowledged that the decision to deny boarding to the passengers stemmed from an “unfortunate series of inaccurate communications, misinterpretations, and misjudgments throughout the decision-making process.”