As the fallout from the violent removal of a passenger from a flight at O'Hare International Airport continued to ripple around the world Tuesday, Mayor Rahm Emanuel moved to contain the damage locally by pledging his aviation commissioner is working with United Airlines to avoid a repeat of the now-infamous incident. 
 
 Chicago aldermen plan to add their voices to the international outcry with a hearing later this week, while questions were raised about how the Department of Aviation police officer involved in the incident handled it and how he will be investigated. 
 
 It's unclear what Emanuel empowered Aviation Commissioner Ginger Evans to do. The mayor declined to take questions on the United issue while dedicating a street sign in honor of Cubs second baseman Javier Baez in Humboldt Park on Tuesday. His office later issued a statement. 
 
 "Anyone who saw that video had the same reaction: this was completely unacceptable at every level," the mayor's statement reads. "I appreciate that Ginger (Evans) took swift action at the Department of Aviation, and her team is already hard at work on a thorough investigation — in partnership with the airline — to ensure nothing like this ever happens again." 
 
 Aviation Committee Chairman Ald. Michael Zalewski, 23rd, said he wants to hear from airport officials and United representatives Thursday about the situation, in which a passenger was dragged from a Louisville, Ky.-bound plane by aviation police Sunday after he refused to give up his seat so an airline employee could sit in it. Cellphone video taken by other passengers and shared online led to worldwide criticism of the airline and the officer. 
 
 The aviation officer who pulled the man out of his seat and down the aisle of the plane has been put on a leave of absence by the city. 
 
 "This has been a black eye for the city. People are talking about this all over the world," Zalewski said. "We need to hear from the Aviation Department and the airline about how to make sure this doesn't happen again." 
 
 Four U.S senators, including the chair of the Committee on Commerce, Science and Transportation, sent several questions about the "very disturbing" incident in separate letters to Oscar Munoz, United's CEO, and Evans. They asked Evans to explain the policies and procedures for aviation officers, and whether they were violated, among other questions. The letter asked for a response by April 20. Additionally Tuesday, Illinois Sens. Dick Durbin and Tammy Duckworth, among other senators, called for United to answer yet more questions about the incident. 
 
 United CEO Munoz, after taking heat for earlier statements that some thought failed to take responsibility, issued an apology Tuesday for the incident. Munoz said the company would review what happened, including how it works with airport security and local law enforcement. 
 
 A frame-by-frame account of security officers forcefully removing a passenger from a United flight the airline said was overbooked. 
 
 "Like you, I continue to be disturbed by what happened on this flight and I deeply apologize to the customer forcibly removed and to all the customers aboard. No one should ever be mistreated this way," Munoz wrote. 
 
 While elected officials decried the way the situation was handled, they offered few specifics about preventing a repeat. 
 
 Emanuel spokespeople did not respond to questions about whether Evans' investigation might include efforts to re-organize or replace the aviation police department. When asked whether it was appropriate for the aviation police officer to come onto the plane and remove the passenger from the flight at the behest of airline personnel, city spokeswoman Jennifer Martinez said in an email that "the matter is under careful review." 
 
 Martinez noted the officer was placed on leave, and added "as our review continues we will not hesitate to take additional action as appropriate." 
 
 The Emanuel administration did not respond to questions about whether the city can require airlines to follow certain rules about how and when they can remove passengers from flights. 
 
 It's unclear, too, how an investigation by the Department of Aviation into its own police would proceed, or what other agencies would get involved. 
 
 While Chicago police are overseen by their own set of investigative and disciplinary authorities, aviation security officers are not part of the Chicago Police Department. The Office of Inspector General Joseph Ferguson has broad jurisdiction to investigate other city employees. 
 
 Reached Tuesday, Ferguson declined to say whether or not his office had launched its own investigation. But when he was asked whether his office had been asked to look into the matter, he said, "No, and it's a little bit surprising, not least because it is being represented that the (aviation) commissioner is conducting what has been referred to as an investigation, and our office is having a hard time getting aviation to say what they're doing by way of an investigation." 
 
 The aviation police are largely unknown to the public at large, with about 300 working at the city's two major airports, sometimes alongside Transportation Security Administration forces and other local and federal authorities. 
 
 Aviation security officers cannot carry weapons but must be state-certified police officers, meaning they have to meet the same minimum standards as other local cops, according to a job description posted on a city website. The officers, for example, have to pass fitness testing and psychological examinations, like Chicago police officers. The officers can "temporarily detain and take people into custody until Chicago police arrives," Martinez said. However, only Chicago police can complete and file an arrest report, she said. 
 
 Amid questions about the aviation police department's protocol and growing public backlash for the incident, an Aviation Department spokeswoman issued a statement Tuesday night. 
 
 "While they do have limited authority to make an arrest, Sunday's incident was not within standard operating procedures nor will we tolerate that kind of action," Karen Pride said in the statement. "That is why we quickly placed the aviation security officer on leave pending a thorough review of the situation." ||||| This Sunday, April 9, 2017, image made from a video provided by Audra D. Bridges shows a passenger who was removed from a United Airlines flight in Chicago. Video of police officers dragging the passenger... (Associated Press) 
 
 This Sunday, April 9, 2017, image made from a video provided by Audra D. Bridges shows a passenger who was removed from a United Airlines flight in Chicago. Video of police officers dragging the passenger from an overbooked United Airlines flight sparked an uproar Monday on social media, and a spokesman... (Associated Press) 
 
 CHICAGO (AP) — The chief executive of United Airlines said the carrier will no longer ask police to remove passengers from full flights after the uproar over a man who was dragged off a plane by airport officers in Chicago. 
 
 In an interview with ABC's "Good Morning America" aired Wednesday, Oscar Munoz said he felt "ashamed" watching video of the man being forced off the jet. He has promised to review the airline's passenger-removal policy. 
 
 Munoz, who leads United's parent company, apologized again to Kentucky physician David Dao, his family and the other passengers who witnessed him being taken off the flight. 
 
 "That is not who our family at United is," he said. "This will never happen again on a United flight. That's my promise." 
 
 In the future, law enforcement will not be involved in removing a "booked, paid, seated passenger," Munoz said. "We can't do that." 
 
 Also Wednesday, a Chicago alderman said representatives from United and the city's Aviation Department have been summoned before a city council committee to answer questions about the confrontation at O'Hare Airport. 
 
 Alderman Mike Zalewski said he did not know who will represent the airline before the Aviation Committee, but Munoz has been notified of the hearing scheduled for Thursday. 
 
 Chicago Aviation Commissioner Ginger Evans will also speak. 
 
 Munoz called the embarrassment a "system failure" and said United would reassess its procedures for seeking volunteers to give up their seats when a flight is full. United was trying to find seats for four employees, meaning four passengers had to deplane. 
 
 It was at least Munoz's fourth statement about the confrontation. 
 
 After the video first emerged, he said the airline was reaching out to the man to "resolve this situation." 
 
 Hours later on Monday, his tone turned defensive. He described the man as "disruptive and belligerent." 
 
 By Tuesday afternoon, almost two days after the Sunday evening events, Munoz issued another apology. 
 
 "No one should ever be mistreated this way," Munoz said. 
 
 The passenger was identified as Dao, a 69-year-old physician from Elizabethtown, Kentucky. 
 
 Attorneys for Dao filed court papers Wednesday asking the airline and the city of Chicago to preserve evidence in the case. Those documents are often the first steps toward a lawsuit. 
 
 Airport officials have said little about Sunday's events and nothing about Dao's behavior before he was pulled from the jet that was bound for Louisville, Kentucky. Likewise, the Chicago Aviation Department has said only that one of its employees who removed Dao did not follow proper procedures and has been placed on leave. 
 
 No passengers on the plane have mentioned that Dao did anything but refuse to leave the plane when he was ordered to do so. 
 
 The event stemmed from a common air travel issue — a full flight. 
 
 At first, the airline asked for volunteers, offering $400 and then when that did not work, $800 per passenger to relinquish a seat. When no one voluntarily came forward, United selected four passengers at random. 
 
 Three people got off the flight, but the fourth said he was a doctor and needed to get home to treat patients on Monday. He refused to leave. 
 
 That's when three Aviation Department police officers boarded the plane. When Dao refused to leave his seat, one of the officers could be seen grabbing the screaming man from his window seat and dragging him down the aisle by his arms. 
 
 Other passengers on Flight 3411 are heard saying, "Please, my God," ''What are you doing?" ''This is wrong," ''Look at what you did to him" and "Busted his lip." 
 
 The U.S. Department of Transportation announced Tuesday that it was reviewing Sunday's events to see if United violated rules on overselling flights. The four top-ranking members of the Senate Commerce Committee asked the airline and Chicago airport officials for more information about what happened. ||||| United Airlines promised to reimburse the 70 passengers on board a plane that a man was dragged off injured after refusing to give up his seat, part of continuing attempts to make amends for his rough treatment. 
 
 United Continental Holdings Inc. Chief Executive Oscar Munoz said the carrier will never again use police to remove passengers from overfull flights. 
 
 “We’re not going to put a law-enforcement official on to remove a booked, paid, seated passenger,” Mr. Munoz said in an interview Wednesday with the ABC television network. “We can’t do that.” 
 
 Do airlines have the legal standing to force passengers off overbooked flights? WSJ's Lee Hawkins has the answer. Photo: AP. 
 
 Mr. Munoz also apologized to Dr. David Dao, the passenger forcibly removed on Sunday from a flight scheduled to travel from Chicago’s O’Hare International Airport to Louisville, Ky. 
 
 “My initial words fell short of truly expressing what we were feeling,” Mr. Munoz said, after more than two days of criticism of his reserved responses. 
 
 The Middle Seat After United, a Look at Your Rights as a Flier 
 
 President Donald Trump said Dr. Dao’s treatment was “horrible.” He said carriers should offer more for vouchers so that passengers willingly give up seats. 
 
 “They should have gone up higher,” he told The Wall Street Journal on Wednesday. “But to just randomly say, ‘You’re getting off the plane,’ that was terrible.” 
 
 United Express Flight 3411, operated by partner Republic Airways Holdings Inc., arrived two hours late on Sunday after United gate agents called police for help removing one passenger whose seat they needed for extra crew. 
 
 The Republic crew were due to reach Louisville on an earlier flight delayed by a mechanical problem, a person familiar with the situation said. A Monday flight from Louisville to Newark, N.J., would have been canceled if they didn’t make it. 
 
 Three passengers left without incident. Dr. Dao refused. 
 
 Stephanie Stamm 
 
 He asked a gate agent who came on board the plane to tell him to leave whether he was being singled out because he was Asian-American, said two passengers on board. 
 
 “I’m a doctor. I have to get back to my patients,” he said, according to Tyler Bridges, a 29-year-old civil engineer seated with his wife five rows in front of Dr. Dao. Jason Powell, a 53-year-old history teacher sitting nearby, remembers Dr. Dao saying that as well. 
 
 O’Hare is patrolled by both the Chicago Police Department and the Chicago Aviation Department. Officers from the aviation department were first to respond to United’s call. 
 
 Mr. Bridges recalls three officers calmly asking Dr. Dao to disembark. “Well, you can then drag me,” video footage posted on TMZ shows him responding. “I’m staying right here.” 
 
 One of the officers pulled Dr. Dao from his seat as he flailed his arms and screamed. 
 
 The Transportation Department sets rules for the compensation airlines have to pay to those involuntarily denied boarding; payouts vary with ticket price. Stephanie Stamm 
 
 “He was trying to do whatever he could to keep them from pulling him off,” Mr. Bridges said. 
 
 Republic crew members came to take the place of the bumped passengers. Other travelers heckled them. “You should be embarrassed to work for this company,” both passengers remember someone shouting. They said Dr. Dao then stumbled back onto the plane, bleeding and appearing to be dazed. 
 
 “I’ve got to get home. I’ve got to get home,” they remember Dr. Dao saying. 
 
 Mr. Powell decided to disembark along with seven students he had escorted on a trip to Europe. He was worried for their safety and offended at how gate agents and police had treated Dr. Dao. “It’s exactly what I’m trying to teach our kids not to do,” Mr. Powell said. 
 
 The airline let passengers disembark so crew could tidy up the plane, Messrs. Powell and Bridges said. 
 
 Related Video A man was dragged off a United Airlines flight from Chicago O’Hare to Louisville, Ky., on Sunday night after agents couldn’t find volunteers to be bumped and selected four passengers at random. A passenger captured video of the ordeal. Photo: Tyler Bridges via Storyful 
 
 A passenger dragged off an airliner, intense social media backlash and competition from lower-cost airlines: WSJ's Lee Hawkins explains United's recent struggles. Photo: Jayse D. Anspach via Storyful 
 
 The aviation department has placed three officers involved in the incident on leave and is conducting its own review. Chicago Mayor Rahm Emanuel condemned the incident but praised the department’s response. “Anyone who saw that video had the same reaction: This was completely unacceptable at every level,” he said. 
 
 A spokeswoman for Dr. Dao’s attorneys didn’t respond to requests for comment. His attorneys said Tuesday that he was recovering in a Chicago hospital. Mr. Munoz said he had reached out to Dr. Dao but hadn’t spoken to him. 
 
 The U.S. Transportation Department is reviewing United’s overbooking policy amid lawmaker calls for changes across the industry. 
 
 U.S. airlines routinely sell more tickets than seats to account for no-shows, and offer compensation to customers who take later flights. They also involuntarily bump thousands of passengers a year, a fraction of hundreds of millions of passengers flown overall. 
 
 But it is rare for those removals to take place on board. For many customers, the altercation on Flight 3411 seemed to confirm their sense that air travel in the U.S. is becoming more impersonal and aggravating. 
 
 “What was once this wanderlust category of adventure now has become torture in many people’s minds,” said Rina Plapler, a partner at brand consultancy MBLM. 
 
 Delta Air Lines Inc., which recently canceled about 4,000 flights after heavy thunderstorms around its Atlanta hub, acknowledged it might have moved some passengers to other flights involuntarily. But the number was small because Delta successfully enticed many to accept lucrative reimbursements. 
 
 “The key is managing it before you get to the boarding process,” said Ed Bastian, Delta’s chief executive, on an earnings call Wednesday. 
 
 Mr. Munoz, who joined United 18 months ago, said that he plans a thorough review of United’s policies for incentivizing passengers to give up seats when flights are overbooked. 
 
 “The use of law enforcement aboard an aircraft has to be looked at very carefully,” he said. “They’re clearly there for a purpose of safety,” said the executive. “But for other reasons, I think that’s a policy we have to relook at.” 
 
 He blamed a “system failure” for the incident, which has dented the carrier’s stock price and led to calls to boycott the airline. The stock shed an early gain Wednesday and ended down 1.1% at $69.93. 
 
 “We have not provided our front-line supervisors and managers...with the proper tools, policies and procedures that allow them to use their common sense,” Mr. Munoz said. 
 
 A spokeswoman for United said the airline “will still reserve the right to call law enforcement for the safety and security of our customers and employees,” for instance if passengers become verbally or physically abusive. 
 
 Mr. Munoz said he hasn’t considered stepping down. 
 
 “I was hired to make United better,” he said. “And we’ve been doing that and that’s what I’ll continue to do.” 
 
 —Shibani Mahtani contributed to this article. 
 
 Write to Susan Carey at susan.carey@wsj.com, Doug Cameron at doug.cameron@wsj.com and Andrew Tangel at Andrew.Tangel@wsj.com
– "This will never happen again on a United flight," the AP quotes Oscar Munoz as saying Wednesday. "That's my promise." The CEO of United Airlines says the company will no longer use police to remove passengers from overbooked flights, the Wall Street Journal reports. That's the policy that resulted in a 69-year-old doctor being violently dragged from a flight earlier this week. “We’re not going to put a law-enforcement official on to remove a booked, paid, seated passenger,” Munoz says. He adds that he was "ashamed" of video showing a bloodied David Dao being removed from his flight. Gate agents for United called officers from Chicago's Department of Aviation to remove Dao. One of those officers has been placed on leave for not following proper procedure, according to the department. Chicago Mayor Rahm Emanuel says the Department of Aviation will be conducting a "thorough investigation" to prevent a similar situation from happening in the future, the Chicago Tribune reports. In addition, representatives from the department are scheduled to answer questions from the Chicago city council on Thursday.