The United Airlines flight that drew worldwide attention after a passenger was dragged from his seat and off the plane was not overbooked, the airline said Tuesday.

United spokesman Jonathan Guerin said the flight was not completely sold out. Instead, United and regional affiliate Republic Airlines – the unit that operated Flight 3411 – came to the conclusion that they had to remove four passengers from the flight to accommodate crewmembers who were needed in Louisville the next day for a “downline connection.”

"They were considered "must-ride" passengers," Guerin told USA TODAY.

RELATED: United CEO says employees followed procedures

United has been under siege since videos of Sunday night's violent struggle on the plane at Chicago's O'Hare Airport went viral. CEO Oscar Munoz late Monday sent a letter to the airline's employees lauding the behavior of the flight crew when a "disruptive and belligerent" Kentucky physician was dragged off a plane in Chicago.

Videos of Sunday's violent confrontation and the bloodied passenger went viral, drawing hundreds of millions of views around the world. Social media outrage rained down on the Chicago-based airline, prompting a public apology Monday from CEO Oscar Munoz. But in the letter sent to employees hours later and obtained by numerous media outlets, Munoz credited employees with following established procedures on Louisville-bound United Express Flight 3411.

Four passengers were selected at random to give up their seats. Three did so. The man literally pulled out of his seat and off the plane was David Dao, a physician in Elizabethtown, Ky.

"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," the letter says. "While I deeply regret this situation arose, I also emphatically stand behind all of you, and I want to commend you for continuing to go above and beyond to ensure we fly right."

Munoz conceded, however, that "there are lessons we can learn from this experience," and he promised an investigation. Chicago aviation officials placed a security officer on leave, saying the incident "was not in accordance with our standard operating procedure."

The incident came two weeks after United took withering criticism for requiring two girls to change out of leggings before boarding a flight in Denver. United explained that leggings and yoga pants are among banned attire for people flying with employee pass privileges because such fliers are viewed as representing the company.