It took less than an hour to kill 20 young children in Connecticut.
That's five times as many as have been killed by guns in the District of Columbia in 12 years.
Nationwide, it has taken four months for the gun deaths of children under the age of 10 to equal the number killed at Sandy Hook Elementary School.
Criminologists say the shootings at the school have stunned the country because they force everyone to confront the rarest and most shocking of deaths: young children killed en masse by a stranger wielding a gun at their school.
"This is an act that is literally unprecedented in the U.S. — the mass slaughter of so many children by one person," said Richard Rosenfeld, a professor of criminology at the University of Missouri-St. Louis. "This is a very, very unusual phenomenon."
The Connecticut shootings "resemble more the acts of terrorists who prey on and kill large numbers of an innocent people than they do the everyday firearm killings," Rosenfeld said.
Since September, 20 children under 10 have been killed by guns in the United States. Eight were accidental and four were domestic killings, while eight were either bystander shootings or homicides by nonrelatives. None involved schools, and only one involved an outsider who killed more than one child.
On Sept. 1, Kahlil Singleton, 8, was fatally shot in Hilton Head, S.C., while playing in his grandmother's front yard. The boy was caught in the crossfire of a neighborhood feud involving several adults. A month later, Jorge Duran III, 3, was shot and killed by his father in the family's apartment near Toledo, Ohio. The boy's mother also was slain. And earlier this month, Ryder Rozier, 3, accidentally shot and killed himself in Logan County, Okla., with a gun he found on a nightstand in his uncle's house. His uncle is an Oklahoma state trooper.
Richard Berk, a professor of statistics and criminology at the University of Pennsylania, said that young children are more at risk of being killed by someone they know than by strangers.
"These stranger killings are horrible, but [children are] more at risk from a family member or someone in their neighborhood," he said.
Although the number of youngsters killed by guns in this country steadily inched upward from 2008 to 2010, last week's massacre of 20 first- and second-graders at Sandy Hook remains a statistical anomaly among rampage killings.
In a study of 62 mass shootings in America since 1982 by Mother Jones magazine, only two cases involved elementary school children shot at their schools. In 1989, at the Cleveland Elementary School in Stockton, Calif., a deranged man killed five children and wounded 29 before committing suicide. In 2006, a man killed five students at a one-room Amish schoolhouse in Pennsylvania. Three of the dead were under age 10.
James Alan Fox, a criminologist at Northeastern University in Boston, said that people shoot young children because "they are the most precious members of society."
"If your intention is to get payback for society being unfair to you, then killing children hurts the most," Fox said.
Between 1999 and 2010, firearms cut short the lives of 1,705 children in the United States under the age of 10, including 21 in Maryland and 49 in Virginia, according to data from the Centers for Disease Control and Prevention.
The deaths of young children from gunshots are dwarfed by fatalities from other causes during that time: 8,499 drownings and nearly 16,000 in traffic accidents.
Among the young victims killed nationwide by guns since 1999, nearly 60 percent were white and 61 percent male. About 1,280 of the deaths were homicides and 380 were unintentional, according to the CDC data. The causes of 45 deaths were undetermined, and two were suicide. Less than one percent of all homicides among school-age children happen on school grounds or on the way to or from school, according to the CDC's School Associated Violent Death Study.
"The vast majority of students will never experience lethal violence at school," the study found.
Although the question of why 20-year-old gunman Adam Lanza opened fire at the school may never be answered, Fox said that if the shooter had a negative experience as a child and wanted to get even with his school, the students were the "surrogates."
"It's oftentimes murder by proxy," he said. "They're linked to the primary target. You can't kill a school but you can kill kids and teachers to get even with the school."
In 2001, 160 children under 10 died by gunfire, the highest annual number between 1999 and 2010. The number dropped to 142 the following year, with a significant decline to 119 in 2003 and 2004, according to the CDC. Sixty-three of the 143 victims in 2006 were preschoolers under 5.
In 2010, the most recent year that the CDC reported the statistics, 155 children under 10 died from gunfire.
An overwhelming number of the gun deaths occurred in the South — 821 — six times as many as occurred in the Northeast, according to the data. Nearly 400 happened in the West and 360 in the Midwest.
Fox said firearm deaths are higher in South, in part, because of relaxed gun laws and the prevalence of guns.
"Gun laws in South Carolina and Georgia are not like they are in Massachusetts and New York," he said. "A lot of guns used in crimes in the North come from the South."
Culture and upbringing also play a role in the rate of gun deaths, Fox said.
"In the South, to resolve a dispute, you take it outside," he said. "In the North, you take it to court."
Bonnie Berkowitz contributed to this report.