Americans hoped never to see another mass school shooting after Columbine, but it kept happening

LOS ANGELES - When the Columbine High School shooting occurred on April 20, 1999 in Littleton, Colorado, it was the deadliest school shooting America had ever seen, and it rocked the country. Many hoped it would be the last.

In the two decades since, shootings have continued to occur at schools ranging from K-12 to college university campuses, causing some to wonder whether there is an epidemic rise in school shooting events. Research from James Alan Fox, the Lipman Family Professor of Criminology, Law, and Public Policy at Northeastern University, shows that there is no such epidemic and that the frequency of school shootings has actually been on the decline since the 1990s.

The intensity, however, of incidents has increased since Columbine; the three deadliest attacks in U.S. history all occurred within the last 12 years.

The following is a timeline of the deadliest school shootings since Columbine.

In the compilation of this list, the FBI definition of a mass murderer (a person who kills four or more people, excluding themselves, in a single incident) was used to inform the selection of the events. Data was collected from long-term studies on school shootings and gun violence by the Cornell Institute for Social and Economic Reform, The Washington Post and Mother Jones.

In the middle of the afternoon on a Monday, 16-year-old Jeffrey Weise murdered his grandfather, a police officer, and his grandfather's girlfriend at their home on the Red Lake Reservation in Minnesota.

He then drove his grandfather's squad car to Red Lake Senior High School where he killed five students, a teacher, and a security guard before turning the gun on himself and committing suicide.

Ten young girls were taken hostage when a 32-year-old milk truck driver and father of three named Charles Carl Roberts IV stormed the West Nickel Mines Amish School on the morning of Oct. 2. He carried a shotgun, a hand gun, and a stun gun.

Roberts ordered the boys and men out of the school, tied the girls up, and then called 911. Roberts threatened to kill the girls "in two seconds" if authorities did not leave the property.

Roberts ultimately killed five of the girls and injured the other five before killing himself.

Thirty-three members of the Virginia Tech community were killed and another 23 injured during the deadliest school shooting in U.S. history.

Seung-Hui Cho had been deeply troubled for years by the time he carried out one of the worst mass shootings in the U.S., according to a summary of the event by The Washington Post. After the Columbine shootings in 1999, Cho received psychiatric treatment at the suggestion of his teachers, who spotted homicidal and suicidal ideation in Cho's writings.

Almost a decade later, on April 16, 2007 at approximately 7:15AM, Cho began his rampage when he fatally shot a student and a resident adviser in a dormitory on Virginia Tech's campus. Before continuing on with the rampage, Cho mailed a package to NBC News containing his manifesto as well as photos and videos of himself. In the materials he sent to NBC, he referred to Columbine shooters Eric Harris and Dylan Klebold as "martyrs."

He then armed himself with two guns and 400 rounds of ammunition and attacked Norris Hall, which houses the Department of Engineering Science and Mechanics, as well as the Center for Peace Studies and Violence Prevention. When police stormed the building, Cho took his own life.

Seven administrators and students were killed and three more injured while on the Oikos University campus. One L. Goh, a 43-year-old former student of the christian nursing school, was suffering paranoid schizophrenic delusions when he stormed onto the campus and opened fire. He later said that he was seeking a refund from the school and believed that administrators were out to get him.

Twenty-six people, 20 of them elementary school-aged children, were gunned down in a matter of minutes by 20-year-old Adam Lanza at Sandy Hook Elementary School in the tight-knit community of Newtown. Before descending upon the school around 9:30 a.m., Lanza killed his mother, who had been a teacher at the school, in her home. Lanza died of a self-inflicted gunshot wound on the scene.

The beach community of Isla Vista near University of California, Santa Barbara saw six people killed and 13 more injured when a 22-year-old man went on a shooting spree.

Elliot Rodger wrote in his journal that he moved to Santa Barbara in the summer of 2011 in search of "love, sex, friends, fun, acceptance, a sense of belonging." He also wrote that if he could not find it, he would enact retribution on those who did enjoy these things.

Three years later, after countless smaller attacks on women and happy couples, Rodger carried out a rampage that left six people dead and 13 injured. He wrote in his journal that, "The plan was to destroy the entirety of Isla Vista." He targeted a sorority house first before driving down the wrong side of the street through the city and firing out his driver's side window. Deputies confronted Rodger at a park, but he managed to drive away before crashing his vehicle into other parked cars.

When deputies reached the car, they discovered Rodger had died of a self-inflicted gunshot.

A group of high school students all received a text from fellow student Jaylen Fryberg, 15, to meet up for lunch in the cafeteria. Two of the students were Fryberg's cousins.

When Fryberg arrived at the cafeteria, he pulled a pistol out of his backpack and targeted the table where they all sat. When a teacher confronted Fryberg, he turned the gun on himself and committed suicide. Fryberg's attack killed one of his peers on the scene, and three more victims would die in the following days, including his cousin Andrew Fryberg.

Fryberg's other cousin was injured, but survived the attack.

Nine people were killed and another nine injured when 26-year-old Chris Harper Mercer brought five pistols and a rifle to Umpqua Community College.

As he carried out the attack, he passed off his writings to a survivor to give to police who later discovered that Mercer had identified with other mass shooters like Elliot Rodger, the perpetrator in the Isla Vista shooting at UCSB the year before.

At the end of the attack, Mercer took his own life.

Shortly before school let out on Valentine's Day, 17 students at Marjory Stoneman Douglas High School were shot and killed and another 17 injured.

Former-student Nikolas Cruz, 19, who had been expelled due to disciplinary problems, returned to the campus armed with an AR-15 style assault weapon. Wearing a gas mask and also carrying smoke grenades, Cruz pulled the fire alarm to draw students and teachers out of classrooms and began firing.

He was captured and arrested one mile away from the school.

Students at Santa Fe High School found themselves under attack when a 17-year-old opened fire on campus. Ten were dead by the time the shooting stopped.

Dimitrios Pagourtzis had taken his father's shotgun and .38 revolver, ultimately killing 10 people and injuring an additional 13.

He was taken into custody after a standoff with law enforcement, who found undetonated explosives on the scene as well.