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(CNN) —  

The number of sexual assaults reported during commercial airline flights is increasing “at an alarming rate,” the FBI said Wednesday, and the number of actual cases could be much higher.

FBI investigations into midair sexual assaults increased by 66% from fiscal year 2014 to 2017. The bureau said it had opened 63 investigations into sexual assault on aircraft in 2017, compared with 57 in 2016, 40 in 2015 and 38 in 2014.

FBI Special Agent David Rodski told reporters the number of sexual assaults during flights is increasing “at an alarming rate,” and added, “We’re not sure why.”

Last year, CNN reported it is difficult to determine just how frequently assaults happen on commercial flights because no federal regulatory agency tracks that data nationwide.

The FBI doesn’t have complete confidence in the official number of midair sexual assaults, because so many cases may go unreported, said Brian Nadeau, assistant special agent in charge with the Baltimore division of the FBI. Investigators believe the numbers are almost certainly much higher.

“I’m shocked at the number of passengers who do not take that act and they’ll wait until the plane is on the ground,” Rodski said. Making sure the proper authorities are notified “allows us to do the investigation, collect witnesses, get the flight crew for statements prior to everyone departing for their next destination.”

Rodski said the bulk of the incidents happen on red-eye overnights, flights of three hours or more where cabin lights might be darkened, and/or instances where alcohol is being consumed.

The FBI told CNN on Wednesday that it does not train flight attendants but is working to raise awareness so the airlines can develop procedures for what to do while in the air. That includes a new “Be Air Aware” poster with the reminder: “Sexual assault on an aircraft is a federal crime.”

The Association of Flight Attendants-CWA, one of the world’s largest flight attendant unions, surveyed nearly 2,000 flight attendants in 2016 about their experiences with midair sexual assaults. Among those who responded, 1 out of 5 said they received a report of passenger-on-passenger sexual assault while working a flight. But according to the survey, law enforcement was contacted or met the plane less than half of the time.

Typically, the association said, intervention comes from flight attendants, but many say their employers haven’t told them what to do if someone says they’ve been harassed or assaulted in flight.

Sara Nelson, a United Airlines flight attendant who is president of the union, told CNN, “In my 22 years as a flight attendant, I have never taken part in a conversation – in training or otherwise – about how to handle sexual harassment or sexual assault.” While policies exist, Nelson says that if they’re not elevated in airlines’ training, flight attendants are at a loss for what to do when confronting inappropriate – and sometimes criminal – behavior.