The head of United Airlines said in an email to his employees Monday that the security guards who violently dragged a passenger from his seat were following “established procedures for dealing with situations like this,” according to a tweet by CNBC reporter Steve Kopack.

“As you will read, the situation was unfortunately compounded when one of the passengers we politely asked to deplane refused and it became necessary to contact the Chicago Aviation Security Officers to help. Our employees followed established procedures for dealing with situations like this,” wrote Oscar Munoz, CEO of United Airlines.

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Munoz's message to staff comes amid public scrutiny after a passenger refused to relinquish his seat on an overbooked plane and was violently dragged off the plane by three security officers.

Videos of the incident have since gone viral.

Munoz opened the letter by saying he too was “upset to see and hear about what happened” Sunday night.

The airline reportedly offered $800 as compensation for individuals who voluntarily gave up their seats, but not enough people offered do so. As a final resort, the airline decided to select passengers to deplane. The man at the center of the incident refused to leave after randomly being selected.

Munoz’s email said that the details of the incident are still being looked into, “especially with respect to why this customer defied Chicago Aviation Security Officers the way he did."

Munoz apologized in a Monday statement for having to “re-accommodate these customers” after widespread outcry about the way the passenger was treated.

“This is an upsetting event to all of us here at United. I apologize for having to re-accommodate these customers,” Munoz said. “Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened.”

“We are also reaching out to this passenger to talk directly to him and further address and resolve this situation,” he added.

The officer who dragged the passenger out of his seat has reportedly been placed on leave.

"The incident on United flight 3411 was not in accordance with our standard operating procedure and the actions of the aviation security officer are obviously not condoned by the Department,” the Chicago Department of Aviation said in a statement on Monday, according to NBC News. "That officer has been placed on leave effective today pending a thorough review of the situation."