When a gunman killed 20 first graders and six adults with an assault rifle at Sandy Hook Elementary School in 2012, it rattled Newtown, Conn., and reverberated across the world. Since then, there have been at least 239 school shootings nationwide. In those episodes, 438 people were shot, 138 of whom were killed.

The data used here is from the Gun Violence Archive, a nonprofit that began tracking school shootings in 2014, about a year after Sandy Hook.

Gunshot Victims in School Shootings Killed Injured Sandy Hook Jan. 2014 Feb. 2014 March 2014 April 2014 May 2014 June 2014 July 2014 Aug. 2014 Sept. 2014 Oct. 2014 Nov. 2014 Dec. 2014 Jan. 2015 Feb. 2015 March 2015 April 2015 May 2015 June 2015 July 2015 Aug. 2015 Sept. 2015 Oct. 2015 Nov. 2015 Dec. 2015 March 2016 April 2016 May 2016 June 2016 July 2016 Aug. 2016 Sept. 2016 Oct. 2016 Nov. 2016 Dec. 2016 Jan. 2016 Feb. 2016 Feb. 2017 May 2017 June 2017 July 2017 Aug. 2017 Sept. 2017 Oct. 2017 Nov. 2017 Dec. 2017 Jan. 2017 March 2017 April 2017 Jan. 2018 Feb. 2018 Parkland, Fla. Gunshot Victims in School Shootings Killed Injured Sandy Hook Jan. 2014 Feb. 2014 March 2014 Apr. 2014 May 2014 June 2014 July 2014 Aug. 2014 Sept. 2014 Oct. 2014 Nov. 2014 Dec. 2014 Jan. 2015 Feb. 2015 March 2015 Apr. 2015 May 2015 June 2015 July 2015 Aug. 2015 Sept. 2015 Oct. 2015 Nov. 2015 Dec. 2015 March 2016 Apr. 2016 May 2016 June 2016 July 2016 Aug. 2016 Sept. 2016 Oct. 2016 Nov. 2016 Dec. 2016 Jan. 2016 Feb. 2016 Feb. 2017 May 2017 June 2017 July 2017 Aug. 2017 Sept. 2017 Oct. 2017 Nov. 2017 Dec. 2017 Jan. 2017 March 2017 Apr. 2017 Jan. 2018 Feb. 2018 Parkland, Fla. Gunshot Victims in School Shootings Killed Injured Sandy Hook July Aug. Jan. 2014 Feb. March April May June Sept. Oct. Nov. Dec. Jan. 2015 Feb. March April May June July Aug. Sept. Oct. Nov. Dec. March April May June July Aug. Sept. Oct. Nov. Dec. Jan. 2016 Feb. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Jan. 2017 Feb. Jan. 2018 Parkland, Fla. Gunshot Victims in School Shootings Killed Injured Sandy Hook Jan. 2014 Feb. March April May June July 2014 Aug. Sept. Oct. Nov. Dec. Feb. March April May June Jan. 2015 July 2015 Aug. Sept. Oct. Nov. Dec. Jan. 2016 March Feb. April May June July 2016 Aug. Sept. Oct. Nov. Dec. Jan. 2017 March April May June Feb. July 2017 Aug. Sept. Oct. Nov. Dec. Feb. Feb. Feb. Feb. Feb. Jan. 2018 Gunshot Victims in School Shootings Killed Injured Sandy Hook Jan. ’14 Feb. March April May June July ’14 Aug. Sept. Oct. Nov. Dec. Jan. ’15 Feb. March April May June July ’15 Aug. Sept. Oct. Nov. Dec. March April May June Jan. ’16 Feb. July ’16 Aug. Sept. Oct. Nov. Dec. Jan. ’17 Feb. March April May June July ’17 Aug. Sept. Oct. Nov. Dec. Feb. Jan. ’18 Parkland, Fla. Source: Gun Violence Archive

Note: Shootings in 2013 are not included because complete data was not available in that year. Months with blanks indicate no shootings archived.

The shootings have taken place at sporting events and in parking lots, cafeterias, hallways and classrooms.

A shooting took place Wednesday at Marjory Stoneman Douglas High School in Parkland, Fla., about an hour northwest of Miami. As of Wednesday night, 17 people had been killed and the number of people injured was unknown.

Sixteen of the 239 shootings shown below can be classified as mass shootings, events in which four or more people are shot.

On average, there have been about five school shootings each month, including episodes that were not mass shootings.

15 school shootings 10 5 Jan. 2014 Jan. 2015 Jan. 2016 Jan. 2017 Jan. 2018 15 school shootings 10 5 2014 2015 2016 2017 2018 15 school shootings 10 5 2014 2015 2016 2017 2018 Source: Gun Violence Archive.

Each of the episodes falls under the definition of “school shooting” used by the Gun Violence Archive.

The nonprofit defines a school shooting as an episode on the property of an elementary school, secondary school or college campus. Another defining characteristic is timing — shootings must occur during school hours or during extracurricular activities.

Only episodes in which people were injured or killed by gunfire are included. Injuries like a leg broken while fleeing the site were not archived.

Police reports often leave out the origin of guns used in school shootings, but researchers say a significant portion of the guns used in such episodes come from the shooters’ homes.

Many children have access to unlocked guns at home. Data from gun ownership surveys conducted by the Pew Research Center show that 54 percent of gun owners with children under 18 living at home say they keep all of their guns locked away.

Fourteen states and Washington, D.C., have laws that impose criminal liability on those who store guns at home and know “or reasonably should know” that those firearms are accessible to children.