The violence one week ago at Sandy Hook Elementary School was horrific, you might say incomprehensible. But mass shootings no longer qualify as unusual in the United States.

According to a list compiled by the Washington Post, there have been 13 this year alone, about one every four weeks.

• Feb. 21: Jeong Soo Paek killed his two sisters, their husbands and himself in a family-owned spa in Norcross, Ga.

• Feb. 27: T.J. Lane, a 17-year-old high school students, is accused of shooting five people, killing three, in a high school cafeteria in Chardon, Ohio.

• March 8: John Shick, a former teaching assistant at Duquense University in Pittsburgh, shot eight people, killing one, as he walked through a psychiatric hospital. He was shot by police.

• April 2: One Goh, a former nursing student at Oikos University in Oakland, is accused of lining people up against a wall and shooting them. Seven people died, three more were wounded.

• April 6: Jake England and Alvin Watts randomly targeted black men as they drove around Tulsa, Okla. on Easter weekend, according to police. Three people were killed, two more were wounded.

• May 30: Ian Stawicki stood up and started shooting after he was asked to leave a Seattle coffee shop. Five people were killed, one was injured. Stawicki later killed himself.

• July 20: James Holmes, a graduate student dressed as a character from Batman, allegedly opened fire during a midnight screening of "The Dark Knight Rises" in an Aurora, Colo. movie theater. Twelve people died, 58 were wounded.

• Aug. 5: Wade Page killed five people and wounded three others shortly before Sunday services in a Sikh temple in Oak Creek, Wis. He then killed himself.

• Aug. 13: Thomas Caffall II, shot two people to death and wounded four others as police tried to serve an eviction notice at his home in College Station, Texas. He was killed by police.

• Sept. 27: Andrew Engeldinger pulled a gun and began shooting people as he was being fired from his job at a sign-making company in Minneapolis. Six people died and three were injured before he shot himself.

• Oct. 21: Radcliffe Haughton was the subject of a restraining order obtained by his wife when he entered the Brookfield, Wis. spa where she worked and shot seven women before killing himself. Three others died.

• Dec. 11: Jacob Roberts, wearing a white hockey mask, began shooting randomly inside a Happy Valley, Ore. shopping mall. Two people were killed and one was injured before he shot himself.

• Dec. 14: Adam Lanza entered Sandy Hook school in Newtown, Conn., and shot 20 children and six adults to death. Lanza also killed his mother and himself. One person was wounded.

In the days since Sandy Hook, our readers have called for stricter gun laws, better mental health programs less glorification of violence in video games and other entertainment. We'll address some of our ideas in an upcoming editorial.

Of course, none of them will ensure there's never another mass shooting. But some lives may be saved, and we owe it to the 80 people killed in mass shootings this year — and hundreds more killed in years past — to try.