United Continental Holdings Inc. drew widespread criticism for having a passenger forcibly removed from a flight, an incident that threatens to further damage the reputation of an airline recovering from a proxy fight and leadership upheaval.

Social media on Monday lit up with videos of a man being dragged screaming off a full flight Sunday evening from Chicago’s O’Hare International Airport to Louisville, Ky. He ended up being treated in a Chicago hospital.

“This is an upsetting event to all of us here at United,” Chief Executive Oscar Munoz wrote in a Twitter post on Monday.

United said it was investigating the incident. It declined to identify the passenger. United said it had asked for four volunteers to leave the airplane because of overbooking. After agents couldn’t attract volunteers to leave in exchange for compensation, United used what it said were its own protocols to choose four passengers to remove. Three complied.

“One customer refused to leave the aircraft voluntarily and law enforcement was asked to come to the gate,” United said.