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, which many critics viewed as perfunctory, even calling Dao "belligerent and disruptive." The statements further fueled the controversy, which began when Dao was videotaped being dragged from an overbooked 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 after the airline failed to get passengers to voluntarily take a later flight. Dao repeatedly refused to leave his seat, prompting Chicago aviation security officers to pull him from it and drag him down the aisle on his back and off the plane. Fellow passengers recorded the incident on their cellphones and video of the incident quickly went viral. Dao was hospitalized Tuesday in Chicago for injuries he sustained while being removed, according to Thomas Demetrio, a Chicago personal injury lawyer Dao has retained. The backlash from the incident spread worldwide, with calls for a boycott of the airline and Munoz's resignation. As of Wednesday morning, an online petition demanding he step down had more than 40,000 signatures. Reaction to the incident was particularly intense among Asian-Americans and in Vietnam. The website of the medical board in the southern state of 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. Reports that he was convicted in 2004 of felony charges related to his medical practice were generally dismissed in Vietnam as a likely 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. After surrendering his medical license and a five-year probationary period, the state licensing board allowed him to practice medicine again in 2015. The security officer who dragged Dao from the plane remains on leave "pending a thorough review of the situation," the Chicago Department of Aviation said. 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. United's share price, meanwhile, has rebounded. It opened at $71.65 Wednesday in New York after plunging to an intra-day low of $68.36 the previous day.