Here's a look at some of the nation's deadliest mass school shootings since 1999:
May 18, 2018:
Students at Santa Fe High School in Texas began to evacuate after fire alarms were activated at the school around 7:45 a.m. after students heard gunfire. Ten people were killed and 14 were injured.
February 14, 2018:
A former student of Stoneman Douglas High School in Parkland, Florida, opened fire on students and staff after activating the fire alarm. Seventeen people were killed and 17 were injured.
November 14, 2017:
A gunman rammed a truck into a gate at the Rancho Tehama Elementary School in Reserve, California, before firing at classrooms. Five people were killed and 18 were injured.
October 1, 2015:
A shooting at Umpqua Community College in Roseburg, Oregon, left 10 people dead and seven wounded. Shooter Christopher Harper-Mercer, 26, exchanged gunfire with police then killed himself.
May 23, 2014:
A community college student killed six people and wounded 13 in shooting and stabbing attacks in the area near the University of California-Santa Barbara campus. Authorities said he apparently shot himself to death after a gun battle with deputies.
December 14, 2012:
In Newtown, Connecticut, an armed 20-year-old man entered Sandy Hook Elementary School and used a semi-automatic rifle to kill 26 people, including 20 first-graders and six adult school staff members. He then killed himself.
April 2, 2012:
Seven people were killed and three were wounded when a 43-year-old former student opened fire at Oikos University in Oakland, California. One Goh was charged with seven counts of murder and three counts of attempted murder, but psychiatric evaluations concluded he suffered from long-term paranoid schizophrenia and was unfit to stand trial.
April 16, 2007:
A senior at Virginia Tech in Blacksburg, Virginia, opened fire in a residence hall and classrooms on campus, killing 32 people and injuring dozens before committing suicide.
October 2, 2006:
A gunman took hostages and shot eight out of 10 girls (aged 6-13), killing five before committing suicide in a Nickle Mines, Pennsylvania, schoolhouse. The West Nickel Mines School was torn down, and a new one-room schoolhouse, the New Hope School, was built at another location.
March 21, 2005:
A 16-year-old shot and killed seven people at Red Lake Senior High School in Red Lake, Minnesota, and wounded five others. The dead included an unarmed security guard at the entrance of the school, then a teacher and five students. The gunman committed suicide.
April 20, 1999:
Two students murdered 12 of their peers and one teacher at Columbine High School in Littleton, Colorado. They injured 21 additional people, and three more were injured while attempting to escape the school. After exchanging fire with responding police officers, the pair of killers subsequently committed suicide.