A shooter opened fire North Park Elementary School in San Bernardino, California, on Monday, killing at least two adults and critically injuring two students, in what is believed to be a murder-suicide.

The attack marks the 220th school shooting since Sandy Hook, when 20-year-old Adam Lanza shot and killed 20 school children and six adults at an elementary school in Newtown, Connecticut, after fatally shooting his mother earlier that morning, in 2012.

This means there’s been an average of nearly one school shooting per week in the U.S. since 2013, according to Everytown for Gun Safety, an advocacy group started by former New York City Mayor Michael Bloomberg. The organization defines a school shooting as, “any time a firearm discharges a live round inside a school building or on a school campus or grounds.” Fifty-three percent of the school shootings between 2013 and 2015 took place in K-12 schools, with the majority in high schools, followed by elementary schools.

School shootings are just part of the gun related violence that takes place in the U.S. Everyday, an average of 93 people are killed from gun violence and over 200 people suffer firearm related injuries. “Nearly 12,000 Americans are murdered with guns every year — a rate more than 20 times higher than that of other developed countries,” Everytown writes. The staggering number of guns in the U.S. only adds to this issue. There are an estimated 270 million guns in the country, which is far more than any other. It’s followed by India, at 46 million, but their population is almost four times that of the U.S.

The Sandy Hook shooting fueled a public debate about gun control, yet five years later, the country faces the same issues. In 2013, a legislation to ban semi-automatic weapons and increase background checks was defeated in the Senate, even though it was widely supported by the public.

Background checks are the “single most effective policy” for gun control, Everytown says and the National Instant Criminal Background check has blocked more than 2.5 million gun purchases since 1998. But since background checks are only required at licensed dealers, buyers often take advantage of this loophole by making purchases at gun shows and on the internet. An estimated 22 percent of guns are acquired this way.

Despite the fact that the U.S. has exponentially higher rates of gun violence than any other Western democracy, the Second Amendment’s “right to bear arms,” makes it extremely difficult or nearly impossible to pass laws on gun reform. Even when former president Bill Clinton passed a law in 1994 that partially banned assault weapons, the political backlash was so severe, Clinton later wrote that he likely, “pushed the Congress, the country, and the administration too hard.”