Federal investigators found that United Airlines violated U.S. Department of Transportation policy when the airline did not provide Dr. David Dao and his wife with a “written oversales notice” after he was forcibly removed from a flight at O’Hare Airport last April.

The department concluded that Dao and his wife never got that notice because Dao was being taken to the hospital, according to a letter sent to United that the Chicago Sun-Times obtained through the Freedom of Information Act.

“We find that United complied with some, but not all, of the requirements of the Department’s oversales rule,” the letter stated.

The incident began when Dao refused to give up his seat aboard the Louisville-bound United Express flight so a working airline crew member could take it. United employees then called police. Three Chicago aviation police officers physically removed Dao, leaving him with a concussion, broken nose and two missing teeth, his attorney previously said.

Cellphone video footage of Dao’s removal went viral. United Airlines and the Aviation Department police came under harsh scrutiny. Three aviation officers were suspended. Dao sued the airline, but eventually settled for an undisclosed amount.

The Department of Transportation made clear that it did not investigate the actions of the officers “because it is not DOT’s role to investigate police conduct.”

After three other passengers were bumped from the flight, United selected Dao and his wife for removal. Once they were told to get off the plane, another woman volunteered to leave, according to investigators.

Citing conflicting witness statements, investigators couldn’t determine if Dao or his wife were given the option to stay on the plane after the woman volunteered.

Investigators also found that of the five people kicked off the plane, only four were given the appropriate “Denied Boarding Compensation” — though the fifth person was given the money he was owed 10 days later.

Typically, the Transportation Department will take measures against airlines that repeatedly flout department policy or regulations. That is not the case with United, according to the letter.

“We generally pursue enforcement action when a carrier exhibits a pattern or practice of noncompliance with the Department’s consumer protection regulations and Federal anti-discrimination statutes that we enforce,” department attorney Blane Workie wrote. “We conclude that enforcement action is not warranted in this matter.”

United did not respond to requests for comment. Dao and his lawyers did not immediately return calls.