Leaders of a key Senate committee have asked United Airlines and Chicago airport authorities to explain what led to Sunday night's forced removal of a man from a United Express flight.

United's explanation "has been unsatisfactory, and appears to underestimate the public anger about this incident," four senators wrote in letters Tuesday to United CEO Oscar Munoz and Ginger Evans, commissioner of the Chicago Department of Aviation.

"The last thing a paying airline passenger should expect is a physical altercation with law enforcement personnel after boarding, especially one that could likely have been avoided," they said.

The senators directed most of their questions at Munoz, including queries about the airline's policy for bumping passengers off oversold flights, and whether it makes a difference that passengers have already boarded the plane, as happened on the United Express plane in Chicago.

The senators said the incident could have been prevented with better communication or "additional incentives" -- an apparent suggestion that United didn't offer passengers enough compensation to voluntarily give up their seats.

The letter was signed by the four top-ranking members of the Senate commerce committee -- the Republican chairman, John Thune, the Republican aviation subcommittee chairman, Roy Blunt, and the two senior Democrats, Bill Nelson of Florida and Maria Cantwell of Washington.

You can view cellphone video of the man being forcibly removed by clicking

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