(NEXSTAR) – The Federal Aviation Administration proposed hefty fines, ranging from $9,000 to $52,500, for four airline passengers who interfered with, and in one incident assaulted, flight attendants between December and February.

The heftiest proposed fine, of $52,500, will be levied on a Delta Air Lines passenger who was traveling from Honolulu to Seattle. According to the FAA, the passenger attempted to open the cockpit door, repeatedly refused to comply with crew members’ instructions and physically assaulted a flight attendant, striking him in the face and pushing him to the floor.

Flight attendants, with the help of a passenger, restrained the individual and placed plastic handcuffs on him. He later broke free of the restraint and struck the flight attendant in the face again.

Police boarded the aircraft and arrested the passenger.

The second heftiest proposed fine, of $27,000, was proposed for a Southwest Airlines passenger going from Phoenix to Chicago.

The FAA said in a press release the passenger began yelling and “forcefully banging his hands on the seat in front of him.” He also allegedly yelled on the flight that he had a bomb and was going to kill someone.

The flight was diverted to Oklahoma City, where the passenger was reprimanded.

The other two incidents, with proposed fines of $18,500 and $9,000, involved a man drinking his own mini alcohol bottles on a JetBlue flight and a woman who refused to wear a mask on an Allegiant air flight.

The names of the passengers have not been released.

According to the FAA, Federal law prohibits interfering with an aircraft and assaulting or threatening to assault aircraft crew or other passengers.

Passengers are subject to both civil penalties and federal criminal fines.

The FAA said it is currently enforcing a “zero-tolerance policy towards passengers who cause disturbances on flights or fail to obey flight crew instructions.”

Over the past year, unruly behavior, including refusing to wear a mask, has gotten over 4,000 people banned by U.S. airlines, according to CBS News.

In January, the FAA said it had ” seen a disturbing increase in incidents where airline passengers have disrupted flights with threatening or violent behavior. These incidents have stemmed both from passengers’ refusals to wear masks and from recent violence at the U.S. Capitol.”

The agency said it had initiated over 1,300 enforcement actions against unruly passengers over the last decade.