White House spokesman Sean Spicer said Tuesday that President Donald Trump had seen what Spicer described as the "troubling" video of a passenger being dragged off a United Airlines plane Sunday. The video has unleashed a global social media firestorm and threats of a boycott. Spicer told reporters at a White House briefing that the incident was "unfortunate" but did "not necessarily need a federal response," adding there were "plenty" of law enforcement agencies available to investigate. United's chief executive, meanwhile, reiterated his support of employees who were involved in the incident, despite the growing backlash surrounding it. CEO Oscar Munoz wrote in a letter to employees Monday evening that the United flight crew "followed established procedures" when the passenger was forcibly removed from the aircraft because it had been overbooked. Because the Chicago-to-Louisville flight was overbooked, the crew asked passengers to voluntarily take another flight in exchange for financial compensation. According to media reports, the airline needed to make room for four employees. No one volunteered, so the airline randomly selected four people, one of whom refused to leave, which was followed by his forced removal by three men identified as Chicago aviation security officers. Video showing the man being dragged from the plane and later returning with a bloodied face was widely circulated on social media, drawing angry reactions. One passenger, Audra Bridges, who posted video of the incident, said the passenger was very upset when he was chosen and explained he was a physician who needed to get home in order to see patients the next morning. Bridges said the man appeared disoriented when he ran back onto the aircraft moments later. Crew members eventually ordered everyone from the plane and did not let them return until the injured passenger had been removed again on a stretcher. Bridges said the passengers were "shocked and appalled" by the incident, which prompted the threats of a boycott as the busy summer travel season begins. The online backlash intensified when Munoz used the word "re-accommodate" in a Twitter posting Monday to describe the forcible removal of the passenger. Munoz, however, also said the airline was reaching out to the passenger "to talk directly to him and further address and resolve this situation." Passenger called 'belligerent' In the letter to employees, Munoz said the passenger "raised his voice and refused to comply" when he was initially asked to leave and became "more disruptive and belligerent" in response to subsequent requests. Crew members had "no choice" except to call Chicago aviation security officers to help remove the passenger, Munoz wrote. In a statement late Monday, the Chicago Department of Aviation said the incident was "not in accordance with our standard operating procedure and the actions of the aviation security officers are obviously not condoned by the department." The statement added one officer involved had been placed on administrative leave pending a review of the incident. Munoz admitted to employees that the airline could learn from the incident but doubled down on his support of his employees' actions. "I emphatically stand behind all of you," he wrote. Leggings incident Sunday's incident followed another controversial occurrence in late March in which two girls, one estimated to be about 10 years old, were prevented from boarding a flight in Denver because they were wearing leggings, a violation of the airline's dress code under a program for United employees. The negative publicity may be adversely affecting the value of the airline. United's stock price dropped nearly 4 percent during late morning trading Tuesday in New York.