While some had speculated that the assault was related to a mask violation, ABC News has learned that it started when the female flight attendant accidently bumped the first class passenger earlier in the flight and quickly apologized.
Later on, the passenger walked to the back of the plane and punched the flight attendant twice in the face, breaking her nose. The passenger then returned to his seat as if nothing happened.
The alleged assault prompted Flight 976 from New York City's JFK International Airport to John Wayne Airport in Santa Ana to make an emergency landing in Denver Wednesday, the airline said in a statement. The plane eventually made it to Orange County.
The airline said the plane landed safely and taxied to the gate where law enforcement removed the passenger and took him into custody. A photo posted on Twitter by another passenger shows the suspect handcuffed and sitting at the gate. Passenger Liv Johnson said she then saw staff duct-tape the passenger to a seat.
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"We are outraged by the reports of what took place on board. Acts of violence against our team members are not tolerated by American Airlines. We are working with law enforcement to support our team member and pursue prosecution, and the individual involved in this incident will not be allowed to travel with us in the future," American Airlines said in a statement.
The head of American Airlines says the incident on Wednesday "is one of the worst displays of unruly behavior we've ever witnessed.
In a video statement posted on social media, American Airlines CEO Doug Parker said it is supporting the injured flight attendant and banning the man accused of assaulting her from flying on the airline again.
"That is not enough," said Parker. "We're doing everything we can to ensure he is prosecuted to the fullest extent possible."
The FBI said it is investigating the incident. The agency said no arrests have been made and its investigation is ongoing. The Federal Aviation Administration also said Thursday that it will investigate the incident.
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The FAA adopted a zero-tolerance policy for unruly passengers on board flights earlier this year. The agency said in August it had issued more than $1 million in fines to unruly passengers in 2021.
One $45,000 fine, announced in August, was against a passenger accused of throwing his luggage at another passenger and, while lying on the aisle floor, "grabbing a flight attendant by the ankles and putting his head up her skirt."
In it's latest update, the FAA said there had been 4,941 unruly passenger reports this year.
A survey by the Association of Flight Attendants released in July found that, of the 5,000 flight attendants surveyed, 85% said they'd dealt with unruly passengers in 2021.
Disruptive passengers had used sexist, racist and/or homophobic language, according to 61%, while 17% said they'd been a victim of a physical attack this year.
CNNWire contributed to this report.