The perpetrator originally claimed he was attacked by the microwave machine which penetrated his body. This recognition was changed to the right answer in the end.
“Alexis wrote that the government had been attacking him for the past three months using “extremely low frequency” electromagnetic waves.70 He wrote: “Ultra low frequency attack is what I’ve been subject to for the last three months . . . . And to be perfectly honest, that is what has driven me to this.”
This is a quote from the report issued by the House oversight committee. As you can see, he was subject to the ultra-low frequency attack for the last three months, ended up with the mass shooting. He realized that the microwave cannot access his brain externally.
The microwave is one type of the electromagnetic wave, whose frequency is identified between 300MHz and 300GHz. On the other hand, the ultra-low frequency is a wave below 300Hz though there are several sub-categories as well. The shooter eventually realized that it was impossible to manipulate him through the microwave but the low-frequency can affect his brain.
As said, he used to be an Aviation electrician’s mate at the US Navy and definitely had some knowledges on the radar. It is a system to search the enemy with the electromagnetic wave which actually uses a microwave. Or, he should have a knowledge of the wireless communication system that the Navy uses a very low frequency to communicate with the submarine, which is just above the ultra-low.
That is partly the reason why he got an idea that the ultra-low frequency was applied to brainwash him. He experienced phenomena which can be explained by the electromagnetic wave directly affecting his brain. The voices were one of them to be heard directly to his brain and there should be more manipulations to control his emotion which was out of control in the end.