- Another passenger said he complained about a passenger's T-shirt that advocated hanging journalists.
- The passenger who spotted the shirt said he was asked if he wanted to take another flight.
- The flight continued with both passengers on board.
A United Airlines customer has complained that the airline didn't remove a fellow passenger who wore a T-shirt that suggested that journalists be lynched.
The passenger, who asked that his name not be used because he said he was worried about receiving threats, sent CNBC a picture of the other traveler who was wearing a T-shirt that said "Rope. Tree. Journalist. Some assembly required." He said he told a flight attendant as he made his way to his seat and said that he considered it a death threat.
The issue shows how flight crews are tasked with assessing offensive or threatening passenger behavior or messages.
Sara Nelson, president of the Association of Flight Attendants, which represents around 50,000 flight attendants across 20 airlines, including those at United, blamed President Donald Trump's attacks on media outlets for new challenges flight attendants are facing.
Friday's episode on board the United flight "is not a focus of our training because no one would have guessed that someone would feel emboldened to do such a thing," said Nelson, who is also a United flight attendant. "It is a direct result of the behavior of the president and those of us on the front lines of aviation are facing new conflicts and dangers at work because of it. It is inexcusable and we condemn the statements made against journalists."
The White House did not immediately respond to a request for comment. Trump, through a spokeswoman, earlier on Monday condemned a doctored scene from the Colin Firth film "Kingsman: The Secret Service" that surfaced showing Trump's face superimposed on actor Colin Firth's body, shooting media outlets and other political opponents.
Airlines have removed travelers from flights before for what they have deemed inappropriate attire. United in 2017 denied travel to two young passengers for wearing leggings because they were traveling on employee passes, which has a stricter dress code.
Under United's contract of carriage states that "passengers who are barefoot or not properly clothed" could be denied travel. It also states that passengers who cause a disturbance that would require one of the pilots to leave the cockpit or if they interfere with crew duties could also be removed from a flight.
The sister of the complaining passenger, Jessica Sidman, a food editor at Washingtonian magazine, tweeted another image, taken on the jet bridge, which was retweeted close to 5,000 times. It elicited more than 3,000 comments, some saying the passenger was exercising free speech and others calling it a threat.
United did not respond to several requests for comment.
Sidman's brother told CNBC via email that another United employee boarded the plane and asked if he wanted to discuss the matter and if he would like to switch to another flight.
"I relayed that the shirt was not just offensive, it was threatening the lives of other passengers," the traveler said. He said Friday's flight, United Flight 824 from Los Angeles to Boston, continued with both passengers on board.