Nineteen students and two teachers 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.

Columbine High School (1999)

Crosses with the names and portraits of the victims of the 1999 Columbine High School massacre are seen at the Chapel Hill Memorial Gardens in Littleton, Colorado File Photo: AFP

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.

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.

Virginia Tech (2007)

Memorials for the 32 shooting victims shot in front of Norris Hall on the Virginia Tech campus. File photo: AFP

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.

Sandy Hook Elementary School (2012)

US President Donald Trump and others listen as Scarlett Lewis, holding a photo of her son, Sandy Hook mass shooting victim Jesse Lewis, speaks during a roundtable discussion about school safety in the White House December 18, 2018 in Washington, DC. Photo: AFP

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.

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.

Marjory Stoneman Douglas High School (2018)

n this file photo taken on March 24, 2018 Marjory Stoneman Douglas High School student Emma Gonzalez(C) listens with other students during the March for Our Lives Rally in Washington, DC. Photo: AFP

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.

Santa Fe High School (2018)

Emergency medical personnel stage in the Santa Fe High School parking lot where 10 students were shot and killed on May 18, 2018 in Santa Fe, Texas. Photo: AFP

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.