Airlines reported nearly 6,000 cases of unruly airline passengers in 2021, according to the Federal Aviation Administration's (FAA) latest figures.
As of Dec. 31, 5,981 passengers were reported for allegedly exhibiting unruly behavior, according to the FAA. About 4,290 of the reported cases were mask-related incidents.
To date, the FAA has launched 1,075 investigations into passenger behavior on flights, which is the highest total since the agency started keeping track in 1995. In the five years from 2016 through 2020, the agency averaged 136 investigations a year.
The number of disruptions on flights began to spike in January 2021, forcing the FAA to adopt stricter penalties for passengers who assault, threaten, intimidate or interfere with airline crew members.
Under the FAA's zero-tolerance policy, violators can face fines of up to $37,000 per violation. However, if a passenger commits multiple violations, they can face an even higher penalty.
The rate of incidents on commercial flights has "dropped sharply" since the agency launched its zero-tolerance campaign.
Still, airlines continue to report cases of passengers allegedly threatening or even becoming physical with the flight crew and fellow passengers.
To try and combat the ongoing rise in incidents, officials from the FAA and Justice Department developed a process for the FAA to send most "egregious" cases to the FBI, which forwards those worthy of prosecution to field offices for investigation.
While the FAA can propose fines against unruly passengers, the agency can't prosecute criminal cases.
The Associated Press contributed to this report.