United Airlines Chief Executive Officer Oscar Munoz took to the airwaves Wednesday in an attempt to quell the outrage over Sunday's forced removal of a passenger, vowing that kind of incident "will never happen again." Facing mounting pressure, Munoz was much more contrite in an interview with ABC News, apologizing profusely to Dr. David Dao and promising that security officers will no longer be used to remove passengers. "We can't do that," he said. The tone of Munoz's remarks contrasted sharply with some of his previous statements criticized as perfunctory – including one in which he called Dao "belligerent and disruptive." The controversy began when video taken by other passengers showed Dao, limp and bleeding from a facial wound, being dragged from a United Airlines flight Sunday at Chicago O'Hare International Airport. Dao was one of four passengers bumped from the flight to accommodate four airline employees. He supposedly was chosen at random after other passengers spurned the airline's offer of cash payments if they would agree to disembark and take a later flight to their destination in Kentucky. Dao refused to leave his seat, prompting flight attendants to call for help from Chicago aviation security officers. Three security guards pulled the Vietnamese physician out of his seat and dragged him down the aisle on his back, toward an exit. Passengers screamed in apparently dismay and shock at the man's treatment; some recorded images of the incident on their cellphones, and that video quickly went viral. Dao was hospitalized in Chicago Tuesday for injuries he sustained while being removed, according to Thomas Demetrio, a personal injury lawyer now representing him. Public outrage over the incident has resulted in calls for a boycott of the airline and online petitions demanding Munoz's resignation. Reaction was particularly intense among Asian-Americans and in Vietnam. The U.S. Department of Transportation is investigating the incident and several legislators have called for new rules to curb the airline industry's practice of overbooking flights. The security officer who dragged Dao from the plane was placed on leave "pending a thorough review of the situation," authorities in Chicago said. The website of the state medical board in Kentucky, where Dao is licensed to practice medicine, shows he graduated in 1974 from the University of Medicine of Ho Chi Minh City in Vietnam. Dao was convicted in 2004 criminal charges involving his medical practice. He surrendered his medical license and observed a five-year probationary period, then was allowed to practice medicine once again in 2015. The 2004 incident was dismissed by many Vietnamese as a smear campaign. "Dr. Dao didn't do anything wrong on that flight and that's the main thing," Clarence Dung Taylor wrote on Facebook, the most popular social media platform in Vietnam.