At least 265 children under the age of 18 picked up a firearm and unintentionally shot themselves or another child, according to a gun control advocacy group.

The new #NotAnAccident Index created by Everytown USA records every such incident on a map and details the story behind each shooting. The total does not include homicides or suicides by minors.

Everytown USA says on their site that the current estimation is low as they are currently looking into at least 30 more shootings. The group says it follows up with local law enforcement in every case they log, in order to find out the type of firearm used and the means of storage for it.

More than two million American children live in homes with guns that are not stored safely and securely, the group says. Last year, it recorded an average of five accidental shootings of minors per week. The majority of the shootings occurred in the victim’s home, and most of the shooters were toddlers or young children who found the firearm while playing. Eighty-three of those shootings resulted in a fatality.

Research by the group finds that basic gun safety and storage—when guns are locked away and kept unloaded—could prevent up to 70% of accidental shooting deaths by children.

Another recent report by NBC News found that at least 555 children under the age of 12 had been killed by guns in the three years after the Sandy Hook school shooting of December 2012.

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Write to Eliana Dockterman at eliana.dockterman@time.com.