What unites mass shootings? It's not an ideology — it's the guns.

A day after Monday’s shooting at The Covenant School in Nashville, we know much more about the shooter and the dead. But one question remains: “Why?” Why this school, why these victims, why was the shooter motivated to take these lives?

The search for a motive is a logical one. There’s a deep desire to understand what pushed a person to carry out such a heinous crime, especially when three children are dead. Nashville police have said that the shooter left behind writings that would potentially showcase their reasoning. But...