(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.

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