Mass Shootings in US Increasingly Common and Deadly

— -- The shooting at a Louisiana movie theater that left two dead victims and nine others injured was a shock to many local residents and politicians but does not qualify as a mass shooting according to federal standards.

In 2013, President Obama signed a law qualifying a mass shooting as one where three or more victims die, meaning that Thursday night's shooting would not count since the third death was that of the shooter.

The FBI has worked from a slightly higher count for more than a decade, as they consider a "serial murder" an incident where four or more victims were killed.

Based off the FBI's definition, which was used by Mother Jones in a study of the frequency of mass murders in the United States, the most recent such incident was the shooting at a military recruiting office in Chattanooga, Tennessee. The Lafayette movie theater shooting took place exactly one week after the Chattanooga shooting, where five service members died.

The Chattanooga shooting came just shy of a month after the Charleston, South Carolina, church shooting that left nine dead.

A much larger gap existed for mass shootings between the Charleston incident and its predecessor, which happened at the Marysville-Pilchuck High School in Washington state in October. That gap marked a 236 day break between mass shootings.

As the regularity of mass shootings increases, the number of fatalities increases as well.

The figures for 2012 are particularly high because it includes the Sandy Hook Elementary School shooting, where 20 children and six adult staff members were killed, as well as the Aurora movie theater shooting, where 12 people were killed and 70 others injured.

Mass shootings that occurred since 1982 tend to align somewhat with population centers, as many took place near the East Coast and along the West Coast.

Similarly, there is a notable gap in the upper Great Plains-Rocky Mountain area.

ABC News' Emily Shapiro contributed to this report.