SAN ANTONIO - United Airlines is facing backlash following a public relations disaster. A video surfaced on Sunday showing security forcibly removing a passenger from an overbooked flight.

Officers dragged him down the isle, and his face seemed to be covered in blood. Tuesday, the airline's CEO Oscar Munoz issued his third apology.

Munoz called the treatment of Dr. David Dao, the passenger in question, "a truly horrific event." He added, "I deeply apologize. No one should ever be mistreated this way."

United's policy said the airline has no rules against overbooking flights. They can decide who to bump. When it comes to making decisions, United said the airline bases them on fare, class, itinerary and frequent flier status.

Overbooked flights are quite common among carriers. According to The Department of Transportation (DOT), it's not illegal.

It's a way airlines ensure they have people to fill their seats in case of cancelations. Airlines set their own guidelines when it comes to overbooking and bumping passengers, but the DOT said overall, fliers have rights no matter who their carrier is.

When there's an overbooked flight where passengers need to be bumped the DOT requires that carriers first ask for volunteers to give up their seat in exchange for compensation. That's usually done before people start boarding the flight.

Involuntary bumping is a little different. The airline is required to get passengers on another flight and to their destination within an hour of their originally scheduled time. Otherwise, the airline has to pay up.

The DOT said passengers are entitled to a full refund up to $675 if the airline can get them to their destination within two hours after their originally scheduled arrival time. If delayed longer, the passenger is entitled to double the amount of their ticket price.

Passengers can also make their own flight arrangements and request an involuntary refund for their ticket.