Fourteen students and a teacher were shot dead Tuesday when an 18-year-old gunman opened fire at their Texas elementary school, the latest in the United States' relentless cycle of school mass shootings.

Here are America's deadliest classroom gun massacres in the last two decades.

A man visits the memorial for victims of the 1999 Columbine High School shooting at the Chapel Hill Memorial Gardens in Littleton, Colorado in April 2019
A man visits the memorial for victims of the 1999 Columbine High School shooting at the Chapel Hill Memorial Gardens in Littleton, Colorado in April 2019 AFP / Jason Connolly

Two teenagers from Columbine, Colorado, armed with an assortment of weapons and homemade bombs, went on a rampage at their local high school.

Twelve students and a teacher were killed during the April 20 massacre. Another 24 people were wounded.

Memorials for the 32 victims of the 2007 Virginia Tech shooting are pictured in front of Norris Hall on the school's Blacksburg, Virginia campus in April 2007
Memorials for the 32 victims of the 2007 Virginia Tech shooting are pictured in front of Norris Hall on the school's Blacksburg, Virginia campus in April 2007 AFP FILES / TIM SLOAN

Columbine, whose name has become synonymous with school shootings, is one of the first -- and still among the deadliest -- such shootings in the United States.

New London, Connecticut residents attend a memorial on December 16, 2012 for victims of the Sandy Hook Elementary School shooting
New London, Connecticut residents attend a memorial on December 16, 2012 for victims of the Sandy Hook Elementary School shooting GETTY IMAGES NORTH AMERICA via AFP / MARIO TAMA

A South Korean student at the Virginia Polytechnic Institute opened fire on the Blacksburg, Virginia, campus, killing 32 students and professors before committing suicide.

Thirty-three people were wounded.

The gunman had apparently idolized the Columbine shooters, referring to them as "martyrs" in a video, part of a hate-filled manifesto he mailed to police during the shooting.

Marjory Stoneman Douglas High School student Emma Gonzalez (C) and her classmates speak during the March for Our Lives rally in Washington on March 24, 2018
Marjory Stoneman Douglas High School student Emma Gonzalez (C) and her classmates speak during the March for Our Lives rally in Washington on March 24, 2018 AFP / JIM WATSON

A 20-year-old man with a history of mental health issues killed his mother in Newtown, Connecticut, on December 14 before blasting his way into Sandy Hook Elementary School.

Activists spread 7,000 pairs of shoes, representing the children killed by gun violence since the mass shooting at Sandy Hook Elementary School in 2012, on the lawn on the east side of the US Capitol in March 2018
Activists spread 7,000 pairs of shoes, representing the children killed by gun violence since the mass shooting at Sandy Hook Elementary School in 2012, on the lawn on the east side of the US Capitol in March 2018 GETTY IMAGES NORTH AMERICA via AFP / CHIP SOMODEVILLA

Twenty children, aged six and seven, were shot dead, as well as six adults. The shooter then committed suicide.

The parents of Sandy Hook victims have led numerous campaigns to toughen gun control laws, but their efforts have largely failed.

Some conspiracy theorists insist the massacre was a government hoax, claiming the shooting involved "actors" in a plot to discredit the gun lobby.

On February 14, a 19-year-old former student at Marjory Stoneman Douglas High School who was expelled for disciplinary reasons returned to the Parkland, Florida, school and opened fire.

He killed 14 students and three adult staff.

Stoneman Douglas students have become crusaders against gun violence under the banner "March for Our Lives," lobbying for tougher gun control laws and organizing protests and rallies.

Their campaign has taken off on social media, mobilizing hundreds of thousands of young Americans.

Ten people, including eight students, were killed when a 17-year-old student armed with a shotgun and a revolver opened fire on his classmates in rural Santa Fe, Texas.

Classes had just started on the morning of May 18 when the shooting began.

Following the tragedy, Texas Governor Greg Abbott unveiled 40 recommendations, mainly focused on increasing armed security on school campuses and stepping up mental health screenings to identify troubled children.

Gun ownership can be a point of pride for many Texans, and even some Santa Fe High School students spoke out against linking the shooting to the need for better gun control.