Last year on Fourth of July weekend, Alexandra Anderson, 13, and her brother, Brayden Anderson, eight, were swimming near a homeowner’s dock on the Lake of the Ozarks in Missouri when they started to scream.

By the time the siblings were pulled from the lake, they were unresponsive and a short while later were pronounced dead.

Two hours later on Cherokee Lake in Tennessee, 10-year-old Noah Winstead, died in a similar manner, while Noah’s friend, 11-year-old Nate Parker Lynam, passed away the following evening.

These were not drowning victims. In all four of these cases, 120-volt AC (alternating current) leakage from nearby boats or docks electrocuted or incapacitated swimmers in freshwater. This little-known and often-unidentified killer is called Electric Shock Drowning or ESD.

“Every one of these deaths was preventable,” BoatUS director of technical services Beth Leonard said. “Any boater and every adult who swims in a freshwater lake needs to understand how ESD happens, how to stop it from happening, and what to do — and not to do — if they ever have to help a victim.”

Click here for the full report by BoatUS.