Sen. Richard Blumenthal (D-Conn.) is calling for a federal investigation of United Airlines after a passenger was forcefully removed from an overbooked flight.

“Yesterday, the world witnessed a disturbing, distressing video of United Airlines having police literally pull a passenger off a flight,” Blumenthal wrote in a letter sent Tuesday to Transportation Secretary Elaine Chao.

The Department of Transportation (DOT) said it is reviewing the incident, but Blumenthal is calling for a more thorough investigation.

“The degrading treatment of this individual is the latest example of a major U.S. airline disrespecting passengers and denying them their basic rights,” Blumenthal said.

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United “bumped” passengers from a flight from Chicago to Louisville, Ky., to make room for crew members who were traveling to another airport. Three passengers, who were already seated on the plane, volunteered to take another flight in exchange for compensation.

But a fourth passenger refused to leave. Other passengers captured police removing him from the plane on video, which went viral.

"This is an upsetting event to all of us here at United,” United CEO Oscar Munoz said later on Twitter. “I apologize for having to re-accommodate these customers. 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.”

But Blumenthal blamed United for “having either oversold seats on the plane or having failed to properly schedule its crew.”

The Transportation Department must take steps to “ensure airlines are not prioritizing their employees over passengers,” Blumenthal demanded. He asked the agency to address the “problem of overbooking passengers throughout the industry.”

Blumenthal also raised concerns about the value of the vouchers offered by United.

"Federal law sets a maximum compensation amount of $1350 for passengers who are involuntarily bumped,” he wrote. “What steps are you taking to ensure passengers get the maximum they are due and are not severely shortchanged at the whim of the airline?”