Premise: In a separate mass shooting on Saturday in Pittsburgh, a man donned a bulletproof vest and killed three police officers after he had been driven into a murderous rage by an argument with his mother over a dog.  According to court papers, Richard Poplawski — who had recently been dishonourably discharged from the US Marines — had been arguing with his mother about the dog urinating inside their home early on Saturday morning when she threatened to evict him. When the argument escalated, she called the police. By the time a patrol car turned up at after 7am, Mr Poplawski, 23, had armed himself with an assault rifle, a long-arm rifle, a pistol and several rounds of ammunition.
Hypothesis: Mass shootings are caused by mothers