Violence like school shootings account for less than 2 percent of all youth homicides, but they "devastate families, schools, and entire communities," writes the federal Centers for Disease Control in a newly published report, and are "unacceptably high."

While the leading cause of death for children are accidents — mostly car accidents — homicide is the second-leading cause of death among U.S. youth age 5 to 18. Among those homicides, violence at school is on the rise, the CDC reports.

"The number of school-associated youth homicides remains unacceptably high," writes the CDC's School-Associated Violent Death Surveillance System report. "The findings indicating that the characteristics of many school-associated homicides resemble youth homicides in the broader community suggest the need for prevention beyond the school setting."

Firearms used in most homicides

Between 1994 and 2016, there were 423 school-related homicides. Of these, 90 percent targeted single victims and most of them were male, the CDC said. That rate remained mostly stable between 1994 and 2016, the CDC said.

But multiple-victim violence killed 30 youth in the same time period — evenly distributed between males and females.

Firearms caused 70.4 percent of youth school-associated homicides. Many perpetrators were younger than 18 years who obtained their weapons from home or a friend or relative. Racial and ethnic-minority adolescents are at higher risk than non-Hispanic white youths. Youth homicide rates are higher in urban areas, too.

"The frequent connections with gang activity and interpersonal disputes suggest that school-associated homicides might often be a reflection of broader community-wide risks," the CDC study says.

Gang influence common motive

In single-victim homicides, gang activity (58.2 percent) and disputes (44 percent) were the most common motives for killing another person when the motive was known. In mass violence, retaliation (39 percent) because of "bullying, rivalry between peer groups, or receiving bad grades from a teacher") was the most common motive, followed by gang activity (34.1 percent) and disputes (29.3 percent).

The most common relationships between perpetrator and single victim were stranger (27.6 percent), rival gang member (23.8 percent), or fellow student (21.2 percent). Multiple-victim killers were strangers (36.2 percent) or schoolmates (36.2 percent) to their victims. Forty percent of single-victim and 60.5 percent of multiple-victim homicide perpetrators who used firearms were younger than 18.

Most violent deaths at school occurred immediately before and after the school day and during lunch. They were more likely to occur at the start of each semester.

Nearly half of perpetrators gave some type of warning signal, such as making a threat or leaving a note, before the event.

Tracking violence with keywords

The School-Associated Violent Death Surveillance System tracks lethal violence in school settings by scanning media online of newspaper and broadcast outlets using keywords "shooting, death, violent, strangulation, beating, attack, stabbing, and died" combined with phrases including "primary or secondary or elementary or junior or high or middle or during or after or grounds or property or playground."

The study included youth homicide victims and no adult homicide victim data.

The surveillance system partnered with the U.S. departments of Education and Justice.