We had a similar situation last year at our outdoor range. We had to shutdown while we modified our range. We ended up installing overhead baffles, and raising our berm up quite a bit. No problems now.
Unfortunately times are changing. Populations are growing and our space is shrinking and more people are aholes. We need to take precautions and protect our freedom to shoot. If you think a range is safe, then double the height of the berm. Stay one step ahead.
I don't put anything past anyone these days, I wouldn't be surprised if the home owners did it themselves from "beyond the range" as the investigators stated. The club has been there since the 60's, how about this, don't by a house near a golf course, airport, farm, or shooting range that has been there for years and then complain about it.