A United Airlines passenger wore a shirt that referenced lynching journalists, worrying other passengers and raising a debate over threats on planes

• A passenger on a United flight complained after he say a fellow passenger wearing a T-shirt that advocated lynching journalists, in an incident that went viral on Twitter and was first reported by CNBC.
• The complaining passenger was given the option of moving to a different flight, but the passenger with the threatening shirt was not confronted and was allowed to fly.
• The episode highlighted the inconsistencies in how airlines enforce dress codes and assess threats on board.
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A United Airlines passenger was boarding his flight from Los Angeles on Friday when he noticed a fellow passenger wearing a threatening T-shirt. But when he complained, the airline didn't remove him.

The passenger's sister, Jessica Sidman, a food editor at Washingtonian magazine, tweeted an image that her brother sent her from the jetway. 

The T-shirt read "Rope. Tree. Journalist. Some assembly required."

CNBC reported that the complaining passenger, who did not wish to be named because he was worried about receiving threats, told a flight attendant about the shirt as he made his way to his seat, saying that "he considered it a death threat."

However, the passenger who complained about the threatening shirt was given the option of switching to a different flight, while the man with the shirt was allowed to stay on board and fly.

Ultimately, both men stayed on the flight to Boston.

The issue reflected the delicate balance that airlines walk when enforcing dress codes and discerning threats from passengers.

United's contract of carriage, which all passengers agree to when they book flights, says that "passengers who are barefoot or not properly clothed" may not be allowed to board flights.

Similarly, flight crews are given broad leeway when it comes to addressing possible security concerns and threats.

However, application of rules has been inconsistent and led to several high-profile incidents in recent years.

In 2017, United made headlines when it denied travel to two young female passengers for wearing leggings; in that instance, they were traveling on employee "non-rev" guest passes, which have stricter dress codes.

In July, American Airlines was criticized after flight attendants forced a Houston-area doctor to wrap a blanket around herself on a flight from Kingston, Jamaica, because they said her summer outfit was "inappropriate."

Last month, two Muslim men said their American Airlines regional flight was cancelled because the crew "didn't feel comfortable" flying with them aboard.

According to CNBC, the t-shirt was available at Walmart until it stopped selling them in 2017.

United did not return a request for comment.