Citing a risk of "serious prejudice" to their client, lawyers want United and the City of Chicago, which runs O'Hare International Airport, to ensure passenger and crew lists, cockpit voice recordings, surveillance footage, internal memos and all other materials related to flight 3411 are not destroyed.
"My initial words fell short of truly expressing the shame", Munoz said today in an exclusive interview on "Good Morning America".
He added that the feelings of embarrassment were "palpable" for him and his United colleagues.
But once police were aboard the plane, it would have been hard to walk away, especially if they did not know why the passenger was asked to leave, said Kevin Murphy, executive director of the Airport Law Enforcement Agencies Network. "We can't do that".
Also Tuesday, Chicago Mayor Rahm Emanuel called the way Dao was treated "completely unacceptable" and praised Aviation Commissioner Ginger Evans for taking "swift action". It is unclear if Dao remains hospitalized.
No one volunteered, despite United's offers of up to $800.
This, the airline said, was in accordance with the airline's contract of carriage.
United was trying to find seats for four employees, meaning it wanted four passengers to leave the plane.
The airline said it is "reaching out" to customers on Flight 3411 and "offering compensation for their flights".
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The announcement from the city's Aviation Department came two days after another officer involved in the Sunday night confrontation was put on leave.
Employees then contacted Chicago Aviation Department officers to help remove Dao.
This news comes after the airline was reported kicking two girls off a flight for wearing leggings last month. But earlier media reports revealed that Munoz, in an internal letter after the incident, blamed the passenger for being "disruptive and belligerent". "The expression of apology", Munoz said.
Some U.S. lawmakers called for new rules that could make it more hard for airlines to overbook flights as a tool for increasing revenue.
No passengers on the plane have mentioned that Dao did anything but refuse to leave the plane when he was ordered to do so.
A spokesman for President Donald Trump says it was "troubling" to watch video of a passenger being dragged off of a United Airlines flight.
United needs a new incentive program to bump passengers who are already seated, rather than using police, the embattled CEO said. "I was not concerned for my safety, nor that of my toddler's or for my pregnancy, until the police were called aboard our plane to remove him".
"I was anxious about what a physical altercation would entail with us sitting directly behind him and if the officers were armed in a tiny, confined space", she said.
The incident involving Dao occurred on Sunday evening in Chicago, aboard a plane bound for Louisville. I have to fix that, and I think that's something we can do.