Some things to know about mass shootings:
HOW IS 'MASS SHOOTING' DEFINED?
There is no concrete definition of "mass shooting." Most researchers who study mass shootings define it as four or more killed (excluding the shooter) in a public place. They do not count domestic violence cases that take place in the home nor do they include gang-related violence. That is also the definition used by the Congressional Research Service.
However, tucked into a law enacted by Congress in 2013 in the immediate aftermath of the Sandy Hook school shooting in Newtown, Connecticut, to provide federal law enforcement assistance for active shooter situations was a description of mass killings as three or more killed.
Some other entities include cases in which four or more people are killed or injured.
ARE MASS SHOOTINGS HAPPENING MORE OFTEN?
Researchers say the number of mass shootings has remained fairly steady over the past three decades with occasional fluctuations. "Some years are worse than others and bad years tend to be followed by not so bad years," said James Alan Fox, a professor at Northeastern University in Boston who has been studying mass shootings since the early 1980s.
However, he and other researchers say it does appear that mass shootings have been deadlier in recent years. Since Sandy Hook, Americans have experienced five of the 10 deadliest mass shootings in modern U.S. history — two of those happening this year alone.
WHAT TYPE OF GUNS ARE USED IN MASS SHOOTINGS?
Researchers say the majority of mass shootings are carried out with handguns.
However, of the 10 deadliest mass shootings in modern U.S. history, half of them had a cache of firearms that included AR-style "assault weapons," one of them included an Uzi submachine gun, and the rest used standard rifles and/or handguns.
WHAT WEAPON WAS USED IN THE SANDY HOOK SHOOTING?
Adam Lanza, the gunman who killed 20 children and six adults at the school in Newtown, used a Bushmaster XM15. That firearm, made by Remington Arms, is an AR-style of firearm that was designed in a way to avoid being among the firearms prohibited by the decade-long "assault weapons" ban.
Relatives of the Sandy Hook victims allege in a lawsuit that the maker of the Bushmaster was negligent by marketing military-style weapons to young people who may be unstable and intent on inflicting mass casualties. The lawsuit against Remington Arms was dismissed because of broad immunity granted to the gun industry, but the Connecticut Supreme Court is weighing whether to reinstate it.