A shooting last weekend at a children’s birthday party in California that left four dead was the 17th mass killing this year — the lowest number recorded since 2006, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University.

What You Need To Know • Seventeen mass killings have occurred this year in the U.S., the lowest since 2006 • Experts say this probably isn't the start of a bigger trend, and is more likely a return to average levels after spikes in 2018 and 2019 • Mass killings, defined as incidents where four or more people are killed, are tracked by a database maintained by The Associated Press and USA Today in partnership with Northeastern University • James Densley, a professor, notes that small changes can appear

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