United CEO promises 'thorough review' of passenger incident The promise was made in an internal email obtained by ABC News.

 -- United Airlines CEO Oscar Munoz has vowed that his company will conduct a “thorough review” of the “truly horrific event” that saw a bloodied passenger forcibly removed from one of the airline's flights Sunday night.

The promise came in an internal company email obtained by ABC News.

The airline chief said that the review would consider, “crew movement, our policies for incentivizing volunteers in these situations, how we handle oversold situations and an examination of how we partner with airport authorities and local law enforcement."

Munoz added: “The truly horrific event that occurred on this flight has elicited many responses from all of us: outrage, anger, disappointment. I share all of those sentiments, and one above all: my deepest apologies for what happened.

“I deeply apologize to the customer forcibly removed and to all the customers aboard. No one should ever be mistreated this way,” the CEO continued. “I want you to know that we take full responsibility and we will work to make it right.”

Munoz said the review’s results would be published by April 30.

Multiple videos of the Sunday incident posted to social media have sparked outrage. In them, the man can be seen being dragged down the aisle of the plane by his arms.

United Flight 3411, operated by Republic Airways, was scheduled to depart Chicago's O'Hare International Airport at 5:40 p.m. local time Sunday, bound for Louisville International Airport.

United told ABC News that it had offered passengers on the plane up to $800 to give up their seats for four crew members who needed to board. No one volunteered, so the airline generated a list of four names to be removed from the flight and re-accommodated, per by the airline's contract of carriage. Three of those people complied, and one did not. That's when the police were called to remove the man.

"The doctor needed to work at the hospital the next day," Twitter user Jayse D. Anspach wrote in a series of tweets accompanied by videos of the incident. "So he refused to 'volunteer.'"

Anspach added, "A couple airport security men forcefully pulled the doctor out of his chair and to the floor of the aisle. In so doing, the doctor's face was slammed against an arm rest, causing serious bleeding from his mouth. It looked like he was knocked out, because he went limp and quiet and they dragged him out of the plane like a rag doll."

The passenger removed from the United plane has been identified as David Dao, a 69-year-old physician from Elizabethtown, Kentucky, specializing in pulmonary disease. In a statement, lawyers for the Dao family said he is at a Chicago hospital undergoing treatment for his injuries.

The tone of Tuesday's email was different from one sent by Munoz a day before. On Monday, Munoz told employees that “there are lessons we can learn from this experience,” but he said employees “followed established procedures.”

In his Tuesday email, Munoz said that the passenger “became more and more disruptive and belligerent,” before he was removed by law enforcement officers.

One of the officers involved was placed on leave Monday. The Chicago Department of Aviation said it did not condone the officer’s actions and that they were not in keeping with standard operating procedure.

"The aviation security officer in question is on paid administrative duty pending an investigation," a spokesperson for the Chicago Department of Aviation said Tuesday.

Meanwhile, New Jersey Gov. Chris Christie asked Transportation Secretary Elaine Chao "to immediately suspend the federal regulation permitting airlines to overbook flights and to remove passengers as a result," according to a release from his office. The airline had initially said that the flight was overbooked, but later clarified that that was incorrect and that the passenger was removed to make room for the crew members needed in Louisville.

"This conduct is abusive and outrageous. The ridiculous statements, now in their third version, of the CEO of United Airlines [display] their callousness toward the traveling public with the permission of the federal government," Christie said. "I know the Trump administration wants to reform regulations to help the American people. This would be a great place to start."

The Department of Transportation is reviewing the incident.

On Capitol Hill, Reps. Peter DeFazio, D-Oregon, and Rick Larsen, D-Washington, asked Chao to provide "any findings from DOT's review of the April 9 incident that occurred on United Airlines Flight 3411," according to a statement. DeFazio is ranking member of the House Transportation and Infrastructure Committee. Rick Larsen is ranking member of the House Aviation Subcommittee.

In a letter to Munoz, a number of Democratic senators said they were "deeply concerned" and asked for more information about the incident. The senators included Brian Schatz of Hawaii, Maggie Hassan of New Hampshire, Dick Durbin of Illinois and Chuck Schumer of New York.