A 20-year-old American Airlines passenger was charged with assault and interference with a flight crew member Monday after he allegedly punched a flight attendant in the face last month, and now faces up to 20 years in jail.

A source told ABC News the alleged attack by Brian Hsu on the flight attendant was not provoked. The source said that the crew member bumped into Hsu, who was traveling in first class, but quickly apologized. Later, the passenger walked to the galley and allegedly punched her two times.

The source added that after the alleged attack that happened on Oct. 27 while flying from John F. Kennedy Airport to John Wayne Airport, Hsu returned to his seat as if nothing had happened. After the alleged incident, the fight was diverted to Denver International Airport.

Hsu's mother Julia Yu was on the plane with him, but she declined to comment after Monday’s hearing, reported CBS Los Angeles. Four witnesses said that the flight attendant told Hsu to stay away from the plane’s bathroom that led him to punch her in the face “with sufficient force to cause her to hit the lavatory door." Another witness “recalled the victim stating, “I have a fractured nose’ after the strike.”

He was released after paying a $10,000 bond, and is scheduled to appear in court in Denver on Nov. 15.

After his Monday hearing, when he was asked by reporters if he hit the flight attendant, he said no. He also claimed that he could not have punched the crew member as he had an injury to his right hand a few weeks ago, which doesn't allow him to make a fist, according to Daily Mail.

Hsu told cops he was going back home after his surgery in Rhode Island to reconstruct his skull, and said that he had suffered a head injury after being assaulted in New York City last year. The injury apparently left him suffering from psychological damage, nausea, loss of balance and "brain fog."

He told authorities that he got up to use the bathroom and he "accidentally" bumped the crew member with his arm or hand when he was standing and stretching in the aisle. He said that the crew member "became agitated and began swinging at Hsu's head" with her hands, so he reportedly became scared because an impact to his head in its present state could cause him severe injury or even death.

He claimed that in order to prevent the flight attendant from hitting him, he backed up towards his seat and defensively raised his hands. Hsu said that the person then charged at him and hit her nose against the palm of his right hand, and after that he did not see any blood or the victim holding her nose.

American Airlines CEO Doug Parker called the alleged attack one of the worst cases of unruly behavior they have ever witnessed, and said, "This type of behavior has to stop."

An American Airlines plane
An American Airlines plane is seen at sunrise on the tarmac of the Reagan Washington National Airport (DCA) in Arlington, Virginia, on April 22, 2021. Photo by DANIEL SLIM/AFP via Getty Images