After the video of a passenger getting dragged off a full United Express flight by airport police sparked outrage, the head of United’s parent company said the airline was reaching out to the man to “resolve this situation.”

By Tuesday afternoon, almost two days after the Sunday evening confrontation in Chicago, CEO Oscar Munoz issued his most contrite apology yet as details emerged about the man seen on cellphone videos recorded by other passengers at O’Hare Airport.

“No one should ever be mistreated this way,” Mr. Munoz said.

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Munoz’s latest statement described the removal as “truly horrific.” He said the company would review policies for seeking volunteers to give up their seats, for handling oversold situations and for partnering with airport authorities and local law enforcement.

An attorney who represents Mr. Dao said his client was being treated at a Chicago hospital for injuries he sustained on the plane and that the family would not comment.

Airport officials have said little about Sunday’s events and nothing about Mr. Dao’s behavior before he was pulled from the jet that was bound for Louisville, Kentucky. Likewise, the Chicago Aviation Department has said only that one of its employees who removed Mr. Dao did not follow proper procedures and has been placed on leave.

No passengers on the plane have mentioned that he did anything but refuse to leave the plane when he was ordered to do so.

Also Tuesday, Chicago Mayor Rahm Emanuel called the way Dao was treated “completely unacceptable” and praised Aviation Commissioner Ginger Evans for taking “swift action.” He promised that a city investigation would “ensure nothing like this ever happens again.”