There have definitely been many damages caused by the electromagnetic wave in the US, some of which were ended up with the mass murder. There is no doubt that the wave created massacres, but the issue is how many of them are triggered by radiowave manipulation.
The manipulator maneuvers the subject to enhance an aggression, to select targets through a delusion and to execute an action by lowering cognitive faculty. That is how the electromagnetic wave manipulates the subject to commit a crime in general. It is necessary to target a society or community for the mass murder on top of these manipulations.
This procedure is not simple, also requiring a physical manipulation to direct the subject minutely, though it is possible to cause a massive killing by mixing up several methodologies.
If the electromagnetic wave is available, they can create any crimes in any countries. There have been many mass murders caused by the radiowave manipulation in Japan, but it is more difficult to kill many people simultaneously than in the US, as a gun possession is basically illegal there. Even if the subject is so deeply framed, it is hard to acquire a weapon to kill many people concurrently in the end.
The car is another way to cause a mass murder, but a physical hurdle is lower at a gun shooting than driving over many victims, which is why the gun possession capability affects the number of victims killed by a murderer. It means there are the same type of many killings occurred in the developed world caused by the electromagnetic manipulation, but the US outnumbers any other countries in terms of the amount of deaths.
There is another exception of the US that the perpetrators are generally shot to death during the criminal activity. It is necessary to control the crime scene but also makes it difficult to investigate why the murder has taken place. This point is quite critical to understand whether the electromagnetic manipulation was conducted or not.
If its methodology is employed, there are some traces of the manipulation including a physical one. Moreover, it is much easier to find the trace if the perpetrator is not killed for a control of the scene. I seriously doubt many cases triggered by the electromagnetic wave, but there were not enough traces of its manipulation in many cases, partly because the murderers were shot dead during the course.