Among other records, the filing says preserving surveillance video, cockpit voice recordings, passenger manifestos, United company policies and incident reports are "crucial and essential" to ensure that Dao's case is not later prejudiced.
Images of the 69-year-old doctor being violently wrestled from his seat and dragged along the plane gangway have prompted outrage around the world, and turned into a PR nightmare for United.
And the Chicago Department of Aviation said three of the officers involved in the incident have been placed on administrative leave. "This will never happen again on a United flight". Video shot by passengers showing the man's bloodied face went viral on social media, prompting a storm of protest.
"The first thing I think is important is to apologize to Dr. Dao, his family, the passengers on the flight, the customers, our employees", he said.
On Wednesday, US senator Chris Van Hollen, a Maryland Democrat, announced plans for the Customers Not Cargo Act, which would prohibit the forcible removal of passengers already aboard an aircraft "due to overbooking or airline staff seeking to fly as passengers".
Dao is scheduled to hold a press conference on Thursday. Users tweeted comments such as; "I heard of "flight or fight" but this is ridiculous", "Putting the Hospital in Hospitality" and also "We'll tell you when to get off".
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Video of the Kentucky doctor, David Dao, being dragged off a United plane at O'Hare is touches nerves.
In a leaked email distributed to employees following the incident, United CEO Oscar Munoz did not apologise to Dao, but instead said that he "refused to comply with crew member instructions" and "defied" security.
"This will never happen again", Munoz said. "You saw us at a bad moment".
United offered passengers $400 and a free night in a hotel to take a flight at 3 p.m. Monday.
It wasn't until Tuesday that Munoz was more contrite. "Currently, they are focused only on Dr. Dao's medical care and treatment". No one volunteered. United chose to select people at random.
Some House Democrats (and New Jersey Gov. Chris Christie, apparently) have even called for a congressional hearing into airlines' ability to intentionally overbook flights.