The number of midair sexual assaults reported on commercial flights is increasing at an “alarming rate,” according to the FBI report on Wednesday. However, investigators predict the actual number of incidences is likely much higher due to the tendency of such cases to go unreported.

FBI investigations into in-flight sexual assaults increased 66% from 2014 to 2017. According to the bureau, 63 investigations into sexual assault on aircrafts were launched in 2017, whereas only 38 investigations were reported in 2014.

FBI Special Agent David Rodski said the majority of cases happen on red-eye overnight flights of three hours or more where cabin lights are often dimmed or instances involving alcohol consumption.

It is extremely difficult to determine just how many sexual assaults take place during flights because no federal regulatory agency tracks that data nationwide.

According to a 2016 report in the New York Times, the only option available to flight crews—aside from informing police—is reporting “disruptive behavior to the Federal Aviation Administration,” which contains “no separate category for sexual assault.”

The Association of Flight Attendants, one of the largest flight attendant unions, polled a group of nearly 2,000 flight attendants in 2016. The findings revealed 1 out of 5 attendants said they received a report of passenger-on-passenger sexual assault while working a flight. According to the same survey, law enforcement was contacted or met the plane less than half of the time.

On Wednesday, the FBI told CNN that it does not formally train flight attendants on sexual assault procedures; however, the agency is working to promote awareness of the situation. The effort includes a new “Be Air Aware” poster with the reminder: “Sexual assault is on an aircraft is a federal crime.”

In addition to a lack of staff training, logistical hurdles can complicate the victim’s decision to speak up.

“If you’re a victim of a crime on the ground, what do you do?” Paul Hudson, president of airline consumer group Flyers Rights, told the Washington Post. “You call 911 and report it to a police officer. But if you’re in an airplane, you can’t do that. You have to report through a flight attendant, and they have to report it to the captain, and the captain has to report it to a ground supervisor for the airline… In many cases, too much time has passed.”

In May, lawmakers proposed legislation to require airlines, rail, and bus operators to report sexual assault or harassment to authorities. The bill is still in committee.