An FBI crime classification report from 2005 identifies an individual as a mass murderer if he kills four or more people in a single incident (not including himself), typically in a single location.
April 20, 1999: Columbine High School. Two seniors at the Colorado High School, Eric Harris and Dylan Klebold, opened fire on students in different parts of the school. The pair committed suicide inside the school in the incident most commonly referenced regarding school shootings.
April 16, 2007: Virginia Tech. In the deadliest domestic shooting incident by a single person in U.S. history, Seung Hui-Cho spread two attacks over about a two-hour period on the Virginia campus. He later killed himself.
April 3, 2009: Binghamton, N.Y. Authorities said Jiverly Wong entered the city’s immigration center and opened fire, focusing much of his attention on a classroom inside the facility. He was later found dead.
Nov. 5, 2009: Fort Hood, Texas. In the deadliest shooting ever at a U.S. military base, police said Army psychologist Nidal Hasan opened fire at the base’s processing center. Hasan was paralyzed from the waist down after being shot by police.
Jan. 8, 2011: Tucson, Ariz. U.S. Rep. Gabrielle Giffords was holding a local event when 22-year-old Jared Lee Loughner shot her in the head and fired on others at the rally. A judge, a Giffords staff member and a 9-year-old were among those killed.
Feb. 27, 2012: Chardon High School. A student later identified by other students but is a juvenile walked up to a table in the cafeteria in this Ohio school and opened fire. A teacher helped chase the shooter out of the building, and he was later arrested by police.
July 20, 2012: Aurora, Colo. During a midnight screening of the newest Batman movie, The Dark Knight Rises, 24-year-old James Holmes is accused of throwing tear gas and opened fire on the crowded theater. Police arrested Holmes at the scene.
Aug. 5, 2012: Oak Creek, Wisc. Wade Michael Page, a 40-year-old Army veteran, walked into a Sikh temple in suburban Milwaukee and opened fire. A white supremacist, Page was wounded by police before taking his own life.
Aug. 13, 2012: Texas A&M. Thomas “Tres” Caffall, 35, shot and killed a man serving an eviction notice in College Station, Texas, near the campus of Texas A&M, which caused an alert on that campus. Caffall then traded gunfire with responding police, and Caffall was killed.
Aug. 16, 2012: LaPlace, La. Two St. John the Baptist deputies were killed and two others were wounded in two separate but connected incidents. After an initial shooting in a parking lot during which a deputy was injured, deputies engaged in a gun battle with suspects, during which two were killed and suspects were apprehended.
Aug. 24, 2012: Empire State Building. Laid-off apparel designer Jeffrey Johnson, 58, shot and killed a co-worker before engaging in a gun battle with police. Johnson was killed in the gunfire.
Dec. 14, 2012: Sandy Hook Elementary School. 20-year-old Adam Lanza fatally shot twenty children and six adult staff members in a mass murder at Sandy Hook Elementary School in the village of Sandy Hook in Newtown, Connecticut.
March 13, 2013: Mohawk Valley shootings, Herkimer County, New York. Kurt Myers, 64, shot six people in neighboring towns, killing two in a barbershop and two at a car care business, before being killed by officers in a shootout after a nearly 19-hour standoff.
April 21, 2013: Pinewood Village Apartment shooting. Federal Way, Washington. Dennis Clark III, 27, shot and killed his girlfriend in their shared apartment, and then shot two witnesses in the building's parking lot and a third victim in another apartment, before being killed by police.
June 6, 2013: Santa Monica rampage. Santa Monica, California. John Zawahri, 23, armed with a homemade assault rifle and high-capacity magazines, killed his brother and father at home and then headed to Santa Monica College, where he was eventually killed by police.
July 26, 2013: Hialeah apartment shooting. Hialeah, Florida. Pedro Vargas, 42, set fire to his apartment, killed six people in the complex, and held another two hostages at gunpoint before a SWAT team stormed the building and fatally shot him.
Sept. 16, 2013: Washington Navy Yard. As many as two gunmen opened fire Monday morning inside the Washington Navy Yard, killing at least 12 people in an attack on office workers at the heavily secured military installation in the heart of the nation's capital, authorities said.