But in a letter to United staff obtained by CNBC on Monday, Munoz said that staff had "followed established procedures" for dealing with overbooked flights and said that the passenger had become "disruptive and belligerent" after refusing to get off the plane.
On Wednesday Dao's lawyers filed an emergency request at the IL state court to require United and the city of Chicago to preserve the recordings, cockpit voice recordings, passenger list, and other evidence pertaining to the incident on Sunday night.
United also said it would compensate all passengers on Dao's flight.
Video recorded by fellow passengers showed Dao on his back as security officers dragged him from the cabin of the parked plane.
He pledged a full review of the circumstances, and said: "No one should be mistreated this way", ABC News reported. Those documents are often the first steps toward a lawsuit.
By Tuesday afternoon, nearly two days after the Sunday evening events, Munoz issued another apology.
Airport officials have said little about Sunday's events and nothing about Dao's behavior before he was pulled from the jet that was bound for Louisville, Kentucky. In most cases, passengers who are bumped from the flight because of overbooking or other reasons are informed at the gate, before they have boarded the plane. A Dept. of Aviation officer was placed on leave after videos of Dao's removal sparked a social media backlash against United. One of the officers involved has since been put on leave, and the department has launched an investigation.
Dao then reaffirmed he would not get up from his seat.
More than 90 percent of passengers bumped from a flight on a major U.S. airline previous year volunteered, according to the U.S. Department of Transportation.