Sacramento massacre shows rising dangers of handguns converted into automatic weapons

As state and federal authorities continue to piece together how so many people were killed and wounded in Sunday’s shooting in downtown Sacramento, their attention has turned to a small, seemingly innocuous culprit: A gun accessory that quickly and cheaply turns many handguns into weapons capable of spraying dozens of rounds with a single pull of the trigger.

At least one of the guns police say were recovered from the massacre that left six people dead and twice as many injured had been modified with a so-called “auto-sear” or “switch” to boost its firepower. The conversion devices are illegal, and authorities say they are turning up at crime scenes across the U.S. with alarming frequency.

Semiautomatic handguns and rifles require shooters to pull the trigger each time they want to fire a bullet. When retrofitted with a switch, however,