DALLAS (AP) — Nearly one in five flight attendants say they have witnessed physical incidents involving passengers this year, and their union is calling for criminal prosecution of people who act up on planes.

A union survey supports what airlines and federal officials have been saying: There has been a surge in unruly passengers this year, who sometimes become violent.

The most common trigger is passengers who refuse to follow the federal requirement that they wear face masks during flights, according to the survey by the Association of Flight Attendants. Alcohol is the next largest factor, with flight delays also playing a role, according to the union.

The union said nearly 5,000 flight attendants responded to its survey from June 25 through July 14 and 85% said they have dealt at least once this year with an unruly passenger. The union said 17% reported seeing a "physical incident," including touching, slapping or striking a flight attendant or another passenger.

Some said they were cursed or yelled at, and some said they were followed through the airport and harassed after the flight ended, said the union, which represents flight attendants at United, Alaska, Spirit and several smaller carriers.

Airlines have banned a few thousand people for the duration of the mask rule, and the Federal Aviation Administration has announced proposed fines against dozens of people. But union President Sara Nelson said more passengers should face criminal prosecution.

"When people are facing jail time for acting out on a plane, we suddenly see some sobering up, and we need some sobering up," Sara Nelson, the union's president, told reporters.

A few cases have led to criminal charges, and crews sometimes ask police to meet the plane when it lands. In May, a 28-year-old woman was arrested on felony charges in San Diego after a video showed a young female passenger punching a Southwest flight attendant in the face. Such cases are usually filed by local prosecutors — the FAA lacks authority to pursue criminal charges.

The FAA said this week that airlines have reported more than 3,600 cases of unruly passengers this year — figures were not kept for prior years. Nearly three-fourths involved disputes over masks. The agency has announced dozens of proposed fines, the largest being $52,500 for a man who tried to open the cockpit door and then struck a flight attendant on a Delta Air Lines flight in December.

The FAA said it is investigating 600 other cases this year — nearly double the number of investigations started in 2019 and 2020 combined. In January FAA Administrator Stephen Dickson announced a "zero-tolerance policy" in which passengers can face immediate enforcement action instead of warnings.