It can sometimes seem as though mass shootings are occurring more frequently. Researchers who have been studying such crimes for decades say they aren't, but they have been getting deadlier.

In the five years since a gunman killed 20 children and six adults at a Connecticut elementary school, the nation has seen a number of massacres topping the death toll from Newtown and previous mass shootings. Many of them involve the same AR-style rifle used in Sandy Hook.

But Americans wanting to know why mass shootings are happening will get few answers. There remains little research on the topic and mass shooters remain an enigma.

It's also unclear whether the higher death tolls are the result of more firearms being available or firearms being more effective.