The drama surrounding the forced removal of a United Express passenger and the response by the United CEO came to a head Tuesday, as senators condemned the incident and demanded a full investigation into the matter.

The Senate Committee on Commerce, Science and Transportation sent a letter Tuesday evening to CEO Oscar Munoz and Commissioner of the Chicago Department of Aviation Ginger Evans evening asking for a conclusive investigation to be delivered by April 20.

Mr. Munoz earlier announced a company investigation to be concluded by April 30.

“The images and emerging accounts of this incident are very disturbing,” the bipartisan committee wrote in its letter. “The last thing a paying airline passenger should expect is a physical altercation with law enforcement personnel after boarding, especially one that could likely have been avoided.”

The committee noted that the Chicago Department of Aviation suspended the arresting officer after the incident, but it requested a full account of policies and procedures and a list of violations.

The committee wrote to Mr. Munoz that United’s explanation of the incident “has been unsatisfactory, and appears to underestimate the public anger about this incident.”

A video showing a passenger being forcefully and violently removed from the plane went viral Monday.

In the video, shot by passengers on Flight 3411 from Chicago to Louisville, Kentucky, on Sunday, a man’s screams are heard as security violently pulls him from his seat into the narrow airplane aisle.

The man has been identified as Dr. David Dao.

Dr. Dao, who said he had patients to see the next day, is dragged out of his seat and into the aisle. He is pulled by security officers by his arms, and there is blood on his face, his glasses are falling off, and his shirt is pulled up over his stomach.

Dr. Dao was one of four randomly selected passengers to be bumped from the flight because of overbooking. The seats were going to airline employees who needed to be in Louisville for the next day’s operations.

The Department of Transportation notes that overbooking is not illegal but it does require airlines to compensate passengers who are moved. Other requirements include asking for volunteers at the check-in or boarding area before randomly choosing passengers to bump. Witnesses reported that United did these and even doubled its compensation offer from $400 to $800, but nobody volunteered to delay their trip.

“This whole trend of overbooking is really awful,” said Lorraine Abelow, president of Abelow PR, who works with airlines but not United. “It’s a policy they should seriously consider revising.”

In addition to the committee-requested investigation, which also sought answers to United’s policy of bumping passengers, Sen. Mark R. Warner, Virginia Democrat, and 20 colleagues signed a letter demanding more answers from United, specifically on its overbooking policy.

“While it is common practice for commercial airlines to sell more tickets than there are physical seats on an aircraft to account for potential ‘no-show’ passengers, overselling tickets can have severe consequences for the traveling public,” the senators wrote.

Adding to the condemnation, New Jersey Gov. Chris Christie called on the Secretary of Transportation Elaine L. Chao to suspend the federal regulation that permits airlines to overbook flights.

“This conduct is abusive and outrageous. The ridiculous statements, now in their third version, of the CEO of United Airlines displays their callousness toward the traveling public with the permission of the federal government,” Mr. Christie, an early Republican supporter of President Trump, said in a statement Tuesday night.

“I know the Trump Administration wants to reform regulations to help the American people. This would be a great place to start,” Mr. Christie said.

The video of Dr. Dao’s violent removal has been viewed hundreds of thousands of times on social media, and the slow response by Mr. Munoz was seen not only as a public relations failure, but detrimental to the business as well.

CBS reported that by Tuesday morning, 340 million Chinese users of the social media platform Weibo were aware of the incident. While Dr. Dao is reported to be Vietnamese, Chinese commenters were calling for a boycott of the airline, saying the incident amounted to discrimination, according to CNN.

United flies to five cities in China, plus Hong Kong and Taipei, and 12 other destinations throughout Asia.

Bloomberg reported that shares dropped by around 5 percent for United Continental Holdings — the parent group of United Airlines — by Tuesday afternoon.

The United CEO’s first response to the incident on Twitter on Monday acknowledged the disturbing video but apologized only for “having to re-accomodate” the passenger.

His response garnered a Twitter backlash for his choice of words with public relations professionals shaking their heads at the blunder.

“This is crisis PR and you need to come right out and say it, not hide behind words,” said a comment by Ms. Abelow, the public relations executive.

Further condemnation came when CNBC reported an internal memo by Mr. Munoz that expressed support for the actions of United employees and security personnel.

He said that because the passenger was “disruptive and belligerent,” crew members were left with no choice but to call Chicago aviation security officers. “I emphatically stand behind all of you,” the memo says.

By Tuesday afternoon, Mr. Munoz was pressured to backtrack on his second statement.

“The truly horrific event that occurred on this flight has elicited many responses from all of us: outrage, anger, disappointment,” the CEO wrote.

“I share all of those sentiments, and one above all: my deepest apologies for what happened. No one should ever be mistreated this way. I want you to know that we take full responsibility and we will work to make it right,” he said.

Moments before United issued that statement, White House press secretary Sean Spicer said the passenger footage showed a disturbing and unfortunate incident but that it didn’t warrant a response from the federal government.

“There’s plenty of law enforcement to review a situation like that, and I know United Airlines has said they are currently reviewing their own policies. Let’s not get ahead of where that review goes,” he said.

“I don’t think anyone looks at that video and isn’t a little disturbed that another human being is treated that way, but again, I think one of the things people have to understand is when there is a potential law enforcement manner for the president to weigh in, pro or con, would prejudice a potential outcome.”

This incident comes on the heels of another public relations disaster for the airline. Last month, two adolescent girls were almost barred from a flight for wearing leggings.

The girls were flying as companions of staff on standby, and United policy dictates that staff and their companions dress business-casual in such circumstances. The girls were given skirts to put over their leggings before they were allowed to fly.

The power of social media to influence a company was on display just last week when Pepsi pulled an advertisement that critics said co-opted protests to sell soda.

The ad featured reality TV star and model Kendall Jenner walking through a protest. She breaks the line between protesters and police to hand an officer a Pepsi, which he accepts with a smile.

Critics slammed the commercial for cheapening and commercializing protest movements such as Black Lives Matter.

“Clearly we missed the mark, and we apologize,” the company said in a statement last week. “We did not intend to make light of any serious issue. We are removing the content and halting any further rollout.”