Here's what scientists know about mass school shootings

The worst mass shooting in the history of the United States took place at a high school in 1999 and was seen as a turning point.

It is now fourth. In the last decade, there have been three school shootings that have killed at least 12 students and one teacher.

As part of our research, we built a database of mass public shootings with over 200 variables, including location and racial profile. Mass public shootings are defined as incidents in which four or more victims are murdered with at least one of those homicides taking place in a public location and with no connection to underlying criminal activity, such as gangs or drugs.

Our database shows that since 1966 there have been 13 such shootings at schools across the U.S, the first being in California in 1989.

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The Robb Elementary School shooting was one of four in which a family member was killed at another location. He shot his grandmother before going to school.

The majority of mass school shootings were carried out by a single person, with only two of them being carried out by two people. At least 182 people were injured in the attacks.

All of the mass school shootings in our database were carried out by men or boys. The average age of those involved in the attacks was 18.

The picture of the shooter in the Robb Elementary School attack has emerged. He bought two military-style weapons just days before his 18th birthday. The shooter is thought to have used a military-style weapon.

Key information on the shooter, including what motivated him to kill, has yet to be released by police. The profile we have built up from past perpetrators is reflected in the picture of the shooter.

Most school shooters have a connection to the school they target. Twelve of the 14 school shooters in our database prior to the most recent attack in Texas were either current or former students of the school. There is no public connection between the latest shooter and Robb Elementary School.

The mass shooting event is intended to be a final act for most perpetrators according to our research and interviews. The majority of mass murderers die in the attack. Seven of the 15 mass school shooters were captured. The only exception was the Robb Elementary shooter, who was shot dead by police.

School shooters leave posts, messages or videos to warn of their intentions.

Some perpetrators are looking for fame and notoriety, inspired by past school shooting. Most school shooters are angry. Their path to violence involves self-hate and despair turned outward at the world, and our research shows they often communicate their intent to do harm in advance as a final, desperate cry for help. The key to stopping these tragedies is for society to be alert to these warning signs and act on them immediately.

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