California shooting marks 17th US mass killing this year - a 20-year low

Experts warn this likely points to regression to mean after recent spike in mass killings rather than continued declineA shooting last weekend at a childrens birthday party in California that left four dead was the 17th mass killing in the US this year - the lowest number recorded since 2006, according to a database that tracks them.The mass killings - defined as incidents in which four or more people are killed in a 24-hour period, not including the killer - are tracked in a database maintained...