From Reason:

Early this morning, New York Times columnist Nicholas Kristof tweeted about last night’s mass shooting at a bar in Thousand Oaks, California, saying, “These kinds of incidents don’t happen in normal countries where there is some reasonable regulation of firearms, starting with universal background checks.” Yet California already requires background checks for all gun buyers, even when the transaction would be exempt from that requirement under federal law. In any case, ABC News reports that the gun used in the attack, a .45-caliber Glock 21 pistol, was “legally purchased,” which means the perpetrator did not have a disqualifying criminal or psychiatric record, as is typically true of mass shooters.