Mother Jones magazine has tracked mass shootings since 1982, including any incident in which at least four people were killed. Its database includes information on the shooting, such as the shooter’s demographic information, apparent motive and weapon of choice.
Below is a graph of all of the incidents that match Thousand Oaks, with at least 10 people killed by the shooter. The horizontal bars show the number of days since the last such incident. The longer the line, the more time had passed.
From 1984 to 2004, there was an incident in which at least 10 people were killed about once every four years. Over the past four years, there have been eight. There have been four such incidents this year alone.
The shooter in Thousand Oaks apparently used a handgun, uncommon for recent shootings. Since the mass shooting at a movie theater in Aurora, Colo., in 2012, most of the incidents in which at least 10 people were killed have included the use of deadlier weapons, such as a semiautomatic rifle (eight of 10 incidents) or a shotgun (four of 10 incidents), often in addition to a handgun (7 of 10 incidents).
Since the assault weapons ban expired in 2004, there have been 14 incidents in which at least 10 people have been killed, eight of them involving the use of a semiautomatic rifle. Three prior to Thousand Oaks involved only a semiautomatic handgun, according to Mother Jones’s data.