(CNN) The story of an 11-year-old boy shooting his grandmother before killing himself is tragic enough. But research suggests children have relatively easy access to firearms in many homes.
It's still not clear how the Arizona boy managed to find his grandfather's gun.
But across the country, millions of children live in homes with at least one loaded, unlocked firearm, researchers say.
Some states have laws that can punish adults if they store guns irresponsibly around children.
But "child access prevention laws" vary widely, and there is no overarching federal law, according to the Giffords Law Center to Prevent Gun Violence.
Here are three key statistics involving children and guns:
An estimated 7% of US children -- or 4.6 million -- live in homes with at least one loaded, unlocked firearm, according to a study published this year in the Journal of Urban Health.
The study, which used data from 2015, said that estimate "is more than twice as high as estimates reported in 2002, the last time a nationally representative survey assessed this."
That's how many states have some form of CAP laws, or child access prevention laws, according to the Giffords Law Center to Prevent Gun Violence. In some of those states, parents can be criminally liable if minors access their guns.
Arizona, where Saturday's shooting took place, is not among the 27 states listed.
But not all child access prevention laws are the same. In fact, there are a lot of variations.
Fourteen states, including California, Texas and Florida, make adults criminally liable if they negligently store guns "where minors could or do gain access," the law center said.
But CAP laws are much different in Utah, which "does not penalize an adult who recklessly or negligently allows a minor access to a firearm," the law center said. Instead, parents are liable if they know their child has a gun but fails to make reasonable efforts to take the gun away.
Many were shocked that the 11-year-old Arizona boy took his life with a gun. But sadly, suicide by gun is not uncommon among children too young to drive.
Suicide by firearms is the 3rd leading cause of injury deaths among children ages 10 to 14, according to the Centers for Disease Control and Prevention. In 2016, 160 children ages 10 to 14 took their lives with firearms.