Van Hollen joined several other senators who collectively wrote a letter to Munoz on Tuesday "to demand answers" about the incident with Dao, the senator said on Twitter.
"The overbooking process makes it possible to use seats on airplanes much more effectively", he said, arguing it helps the customer by allowing them to book on a sold-out flight that's unlikely to remain that way and the airline by helping ensure that all flights are at capacity. "We can't do that".
The City of Chicago is in charge of O'Hare International Airport, which is where the plane was sitting when the incident happened.
David Katzman, an aviation lawyer with Katzman Lampert & McClune, said that standard doesn't likely apply in the United case.
"They roughed him up, and the reason they were overbooked is because they were trying to get four United Airlines employees on board", he revealed. Munoz also received $6.8 million in stock that will vest over three years, and he can get an additional bonus worth at least $10.5 million.
After the video first emerged, he said the airline was reaching out to the man to "resolve this situation".
The incident became a public-relations disaster for the airline company. An internal memo seen by NBC News also appeared to lay the blame on the "disruptive and belligerent" Dr Dao.
United Airlines' CEO Oscar Munoz offered his "deepest apologies" to Dao.More news: Tip-in by Ryan lifts Senators over Bruins 4-3 in overtime
"We are not going to put a law enforcement official to take them off there", Munoz said.
Lawyers acting for Dr David Dao have filed an emergency request with an IL court requiring the carrier to preserve video footage and other evidence relating to the incident, which sparked outrage on social media. Attorneys for Dao have filed court documents requesting all video, audio, reports and personnel files of the officers be preserved presumably for possible future litigation.
His representatives are expected to hold a press conference on Thursday detailing his experience.
The event stemmed from a common air travel issue - a full flight.
United offered passengers $400 and a free night in a hotel to take a flight at 3 p.m. Monday. He refused to leave. The officers receive more training and pay than private security guards, but not as much as members of the Chicago Police Department.
"Sir, we'll have to drag you", the Aviation security officer can be heard saying.
The Department of Transportation's Office of Aviation Enforcement and Proceedings said in a statement Tuesday that it had begun reviewing what happened on the flight.