One of the officers who dragged a passenger off a United Airlines flight did not follow standard operating procedures and has been placed on leave, according to the Chicago Aviation Department.

It was not immediately clear which of the three men seen in a viral video taken by another passenger was placed on leave.

The department, which did not name the officer, said it did not condone the aviation security officers’ actions Sunday night.

Meanwhile, the CEO of United Airlines’ parent company said the airline is conducting its own probe of the incident.

United Continental Holdings Inc. CEO Oscar Munoz said in a statement that the episode is “upsetting to all of us here at United.”

Munoz said he was reaching out to “talk directly” with the man.