"He was a really sweet man," John Klaassen, a teacher who was traveling on a school trip in Chicago, told CNN's Don Lemon in an interview April 10.
Klaassen told Lemon that the 69-year-old man, who has not been identified, chatted with some of his students before being told his seat was given to an airline worker.
At that point, the man became inconsolable and repeatedly refused to leave. He was forcibly removed by police and then taken to the hospital with injuries from the confrontation.
Video of the incident has ignited a firestorm online and led to criticism and boycotts of United. The officer shown in the video has been placed on leave as the government reviews the case.
The airline has since apologized.
"This is an upsetting event to all of us here at United," CEO Oscar Munoz said in a statement. "I apologize for having to re-accommodate these customers."