The passenger who was violently dragged from his seat on a United Airlines flight from Chicago's O'Hare International Airport signalled on Wednesday (12 April) that he will pursue legal action against the airline or the city of Chicago. David Dao is seeking to have Cook County Circuit Court save all records relating to the incident.

Dao filed an emergency "bill of discovery" against United and the city of Chicago to keep all video, cockpit recordings and other reports from the flight, as well as the personnel files of the aviation department officers who pulled Dao off the plane. Dao, a 69-year-old doctor, has also hired Thomas Demetrio, a high-powered personal injury lawyer, according to a statement from Demetrio.

Court documents state that Dao "currently has no access to them and believes that serious prejudice" will affect his case if these records are not saved, NBC News reported.

Dao was one of four people randomly selected to get off the overbooked flight before it was set to take off from O'Hare International Airport to Louisville. Passengers were asked to leave the flight to make room for United flight crew members who needed to travel to their next assignment.

After Dao refused, saying he had patients to see the next day, United called in Chicago Aviation police officers to forcibly remove him from the plane. Fellow passengers captured footage of the officers seemingly slamming Dao against an armrest before dragging him along the floor down the aisle and off the plane.

The mobile phone footage of the struggle went viral and has caused a PR nightmare for the airline.

Airline CEO refuses to quit

The request came less than two hours after United CEO Oscar Munoz issued an apology for the incident. Munoz told Good Morning America that he felt "shame" while watching the video of Dao being forcibly removed from the plane. However, in an internal letter to the company, Munoz had earlier accused the doctor of being "disruptive and belligerent".

Munoz said his initial response "fell short" of expressing how he felt. He promised that the incident would "never happen again on a United Airlines flight".

Despite calls for Munoz's ousting, the United CEO said he will not step down."I was hired to make United better, and we've been doing that. And that's what I'll continue to do," he said.

According to NBC News, three of the officers involved have been placed on administrative leave.

President Donald Trump commented on the situation on Wednesday, calling it "horrible". In an interview with the Wall Street Journal (WSJ), Trump suggested the incident could have been avoided if the airline had offered bigger vouchers for volunteers.

"You know, there's a point at which I'm getting off the plane ... seriously," Trump said. "They should have gone up higher. But to just randomly say, 'You're getting off the plane,' that was terrible."