AA Flight 976 left JFK Airport at 4:28 p.m. for John Wayne Airport in Orange County when the passenger reportedly punched the attendant in the nose.
The plane landed safely in Denver at 6:38 p.m. Mountain time, according to FlightAware. The attacker was arrested.
"American Airlines flight 976 with service from New York (JFK) to Santa Ana (SNA) diverted to Denver (DEN) due to a passenger who physically assaulted a flight attendant. The aircraft landed safely and taxied to the gate where law enforcement removed and apprehended the passenger," the airline said in a statement.
A photo of the detained suspect sitting in a chair at the airport in Denver was shared by Mackenzie Rose. He reportedly assaulted the flight attendant over his refusal to wear a mask.
"We are outraged by the reports of what took place on board. Acts of violence against our team members will not be tolerated by American Airlines. We have engaged local law enforcement and the FBI and we are working with them to ensure they have all the information they need. The individual involved in this incident will never be allowed to travel with American Airlines in the future, but we will not be satisfied until he has been prosecuted to the full extent of the law. This behavior must stop, and aggressive enforcement and prosecution of the law is the best deterrent."
Airlines reported seeing a spike in bad behavior among people flying despite a zero-tolerance policy, with 1,300 reports of unruly passengers made from February to this past May, said the Federal Aviation Administration.
"We thank our crew for their quick action and professionalism to ensure the safety of their fellow team members and customers on board. Our thoughts are with our injured flight attendant and ensuring that she and her fellow crew members have the support they need at this time," said AA in a statement.