[[[To believe this requires one to believe that the Walmart shooter did not act alone.]]]
Maybe someone had a manifesto ready to go if and when another attack happened. This thing seems so in depth and organized. Not the dead end rantings of a homicidal lunatic.
Or as is said, maybe he didn’t act alone or had a handler.
If you remember the San Bernardino massacre, you will remember the FBI knew who the shooter was even before the last body hit the floor. He was on their radar. It was predictable.