The airline faced a wave of backlash on Monday, after videos showed officers dragging the passenger off United flight 3411 because it had been overbooked. The man also sustained injuries to head when he struck an armrest while being carried out.
"This situation was unfortunately compounded when one of the passengers we politely asked to deplane refused and it became necessary to contact Chicago Aviation Security Officers to help," CEO Oscar Munoz said in email to employees, reported by CNBC. "Our employees followed established procedures for dealing with situations like this. While I deeply regret this situation arose, I also emphatically stand behind all of you."
In an earlier statement, Munoz apologized for "having to re-accommodate these customers" and called the incident an "upsetting event."
Read more: Should Overbooking Flights Be Illegal?
In recounting the sequence of events, Munoz told employees that the passenger "refused" to deplane and "became more and more disruptive and belligerent" and faulted him for "running back onto the aircraft in defiance of both our crew and security officials."
One of the officers involved in the incident was placed on leave Monday.
"Treating our customers and each other with respect and dignity is at the core of who we are, and we must always remember this no matter how challenging the situation," Munoz added in his email to employees.