David Dao, the Elizabethtown doctor who was yanked off an overbooked United Airlines flight Sunday, has had a troubled history in Kentucky.

Dao, who went to medical school in Vietnam in the 1970s before moving to the U.S., was working as a pulmonologist in Elizabethtown when he was arrested in 2003 and eventually convicted of drug-related offenses after an undercover investigation, according to documents filed with the Kentucky Board of Medical Licensure last June. The documents allege that he was involved in fraudulent prescriptions for controlled substances and was sexually involved with a patient who used to work for his practice and assisted police in building a case against him.

Dao was convicted of multiple felony counts of obtaining drugs by fraud or deceit in November 2004 and was placed on five years of supervised probation in January 2005. He surrendered his medical license the next month.

The Kentucky Board of Medical Licensure permitted Dao to resume practicing medicine in 2015 under certain conditions.

RELATED: Man forcibly removed from United flight

Dao was removed from United Express Flight 3411, bound for Louisville, on Sunday night at O'Hare International Airport in Chicago in an incident captured on video that has gone viral.

In the video taken by passengers, Dao refuses to be give up his seat. He then screams as three Chicago Aviation officers begin pulling him from his seat. Dao's head can be seen striking an armrest before he is dragged down the aisle by his arms, seemingly unconscious.

As he is dragged, some passengers can be heard admonishing the security officers.

Dao, his wife and two other passengers were asked to leave the aircraft because the flight was full and four crew members needed their seats. The airline had offered vouchers worth up to $800 for passengers to give up their seats, but no one took the offer. Four passengers, including Dao, were then selected to be bumped.

United has apologized for the incident. At least one of the security officers has been placed on leave, according to authorities. The U.S. Transportation Department also is investigating whether United complied with federal regulations regarding overbooking.

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