CHICAGO—An attorney for the passenger dragged off a United Airlines flight this week said his client was preparing to sue and that his treatment by United reflected the plight of many airline customers across the U.S.

“For a long time, airlines—United, in particular—have bullied us,” said attorney Thomas Demetrio, who is representing the passenger, David Dao, a 69-year-old physician. “You don’t treat the people who help make you the corporate entity you are like Dr. Dao was treated.”

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