A United Airlines jet takes off from Washington National Airport in Washington, US, August 9, 2017. [Photo/Agencies]

CHICAGO - The US Department of Transportation (DOT) will deal no punishment to the United Airlines (UA) over its passenger-dragging scandal that happened at Chicago O' Hare International Airport in April this year.

DOT has made this clear in a letter sent to the airline in May this year. And the letter remained unknown until non-profit organization Flyers Rights obtained it under the Freedom of Information Act (FOIA) and released it Wednesday, President of Flyers Rights Paul Hudson told Xinhua Thursday.

DOT Assistant General Counsel Blane Workie wrote in the letter "there was no evidence that United discriminated against any of the passengers who were involuntarily denied boarding on Flight 3411 based on their race, color, national origin, religion, sex, or ancestry."

"We generally pursue enforcement action when a carrier exhibits a pattern or practice of noncompliance with the Department's consumer protection regulations and Federal anti-discrimination statutes that we enforce," Workie wrote.

In his email to Xinhua, Hudson said "it should be clear that removing Dao from Flight 3411 in such a brutal manner and without legal notice was egregious in every sense of the word." DOT's conclusion that UA's conduct did not warrant an enforcement action is a dereliction of duty, Hudson said.

Dr. David Dao was forcibly dragged from an overbooked UA flight on April 9, a scene that has been captured by fellow passengers in their cell phones and circulated online and went viral, triggering nationwide condemnation.

It took 107 days for DOT to respond to Flyers Rights' FOIA Request, according to a newsletter from Flyers Rights.

UA has reportedly reached a settlement with Dao.