(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. 
 
 Read More ||||| Be Air Aware: Sexual Assault Aboard Aircraft Poster 
 
 Awareness poster cautioning the public to Be Air Aware and that sexual assault aboard an aircraft is a federal crime that should be reported to the flight crew and the FBI.
– "Sexual assault on an aircraft is a federal crime," the FBI warns in its new Be Air Aware campaign—and it's a crime that the agency says has increased "at an alarming rate" in recent years. The bureau says there has been a steady rise in reported mid-air incidents, with the 38 investigations it opened in 2014 rising to 63 last year; the true number of incidents could be much higher, reports CNN. The FBI says it's not sure what's behind the increase, but it urges passengers to "hit that call button" and notify the flight crew immediately when there is an incident. Too often, passengers either fail to report the assault or only report it long after the flight, when it is harder to investigate and prosecute offenders, the bureau says. The FBI says sexual assault is more common on long-haul, overnight flights and often happens when a passenger is asleep. Brian Nadeau, assistant special agent in charge of the Baltimore division, tells the Washington Post that alcohol is often involved, and it is common for victims to be in either the middle or window seats. "We find offenders will often test their victims, sometimes brushing up against them to see how they will react or if they will wake up," Nadeau says. "Do not give these offenders the benefit of the doubt." (This airline has created a service to help women flying alone.)