Chicago's inspector general on Tuesday confirmed earlier reports that the officers involved had suggested that it was Dao's fault that he struck his face on an armrest before he was dragged off the plane. One of the officers yanked Dao out of the seat when he refused to leave, and dragged him down the aisle.
Airport security officials who were caught on video in April forcibly removing a passenger from a United Airlines flight in Chicago have been disciplined. The inspector general's report said "the investigation also uncovered that the employees made misleading statements and deliberately removed material".
The investigation also identified "significant confusion" regarding the role of unarmed aviation security officers, and blamed the Chicago Department of Aviation for a "fundamental failure to implement practical policies and procedures".
A statement from officials said the Chicago Department of Aviation (CDA) employees mishandled a non-threatening situation that led to a forcible removal of a passenger.
The incident left Dao with injuries that his attorneys describe as a broken nose, two chipped teeth and a sinus problem that will require surgery.
The fiasco became a huge public relations headache for United. The scene had been filmed by other passengers, made the rounds of the web and led to a wave of worldwide condemnation.
He was later filmed standing up, saying over and over "just kill me".
After initially mishandling the aftermath, the airline repeatedly apologized, settled with Dao for an undisclosed sum, and announced a series of operational changes.
United has stopped asking passengers already seated on planes to give up their seats and raised the maximum amount it can offer passengers who voluntarily give up their seats from $1,350 (£1,024) to $10,000 (£7,580).