CHICAGO (CBS) — The U.S. Department of Transportation has decided not to fine United Airlines for having a doctor dragged off a plane in April, when he refused to give up his seat to make way for airline employees, according to a letter obtained through a FOIA request.

It is an image that has been replayed many times over: Dr. David Dao being dragged off a United flight at O’Hare International Airport on April 9, his face bloodied after the airline asked Chicago Department of Aviation officers to remove him from the plane.

Dao suffered a concussion and a broken nose, and lost two teeth when his head hit an armrest as security officers yanked him out of his seat.

The U.S. Department of Transportation sent United a letter in May – a month after the incident – to tell the airline that even though it had certain “failures” that day, it was off the hook, and wouldn’t have to pay a fine.

“This and other airlines have been fined in the past for violating some of the same bumping rules. I’m really amazed,” said Paul Hudson, president of the organization Flyers Rights, which issued a Freedom of Information Act request to get the government’s letter to United.

Hudson said the U.S. Department of Transportation’s conclusion is shocking.

“Some people have suggested, well, they may have given them a whitewash, because he presumably got a large monetary settlement,” he said. “That’s also not any good excuse; no more than if you’re the victim of a crime and there’s a parallel civil case. It shouldn’t affect the official investigations in any way.”

United Airlines agreed to a settlement with Dao for an undisclosed amount. The settlement also absolved the City of Chicago of any liability.

The airline also publicly apologized for the incident, vowing it will never happen again.

United has promised to offer passengers up to $10,000 in the future to give up their seats if a flight is overbooked or airline employees need a seat on the same flight. The airline also said it no longer will require customers to give up their seat once they have boarded the plane, unless there is a safety or security risk involved.