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Federal officials decided not to punish United Airlines for dragging a passenger off an overcrowded plane.

The Transportation Department said it had found no evidence that the airline violated David Dao's civil rights in the now-infamous April 9 incident in Chicago. There was also not enough evidence that the airline had violated rules regarding bumping passengers, the department said.

A lawyer for the department told United about the decision in a May 12 letter but didn't make the matter public. An advocacy group, Flyers Rights, released the letter on Wednesday after obtaining it through an open-records request.

Paul Hudson, the president of Flyers Rights, criticized the decision and questioned how the Transportation Department could conduct an investigation so quickly. He called the manhandling of the 69-year-old Dao "egregious in every sense of the word."

RELATED: David Dao and United Airlines Reach ‘Amicable’ Settlement After Viral Video Incident

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The incident started when United agents called O'Hare Airport security officers for help in making room on a United Express plane for four employees who were traveling to staff a flight the following morning in Louisville, Kentucky.

Video of Dao being yanked from his seat and dragged down the aisle was viewed millions of times.

Through our FOIA request, we received the letter from DOT to United Airlines re the Dr. Dao case: https://t.co/E6mAbdhOoE — Flyers Rights (@FlyersRights) September 6, 2017

In the two-page letter to United, the Transportation Department's assistant general counsel, Blane Workie, said the agency takes action when an airline repeatedly or egregiously violates consumer-protection laws. She said United fixed one mistake in calculating compensation for another passenger, and failed to give Dao and his wife a required written notice of their rights only because they had left the airport to seek medical help.

"Therefore, we conclude that enforcement action is not warranted in this matter," Workie concluded.

She said the agency found no evidence that United discriminated against Dao, who is Asian-American, on the basis of race.

United avoided a lawsuit by reaching a settlement with Dao a few weeks after the incident. Terms of the settlement were not disclosed.

The CEO of United Continental Holdings Inc., Oscar Munoz, apologized for initially defending the airline's handling of the incident and blaming Dao, who lost teeth and suffered a broken nose and a concussion.

The airline apologized for the incident again Wednesday and said it has made changes to reduce overbooking.

"This incident should never have happened, and we are implementing all of the improvements we announced in April," spokeswoman Megan McCarthy said in a statement. "While we still have work to do, we have made meaningful strides" and have reduced the bumping of passengers nearly 90 percent since May 1, compared with the same period last year."

Airlines are allowed to oversell flights. When they do, they typically offer travel vouchers to encourage some people to give up their seats. They can also force passengers off a flight, but there are rules, including compensation.

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