On Sept. 4, 14-year-old Colt Gray shot and killed two students and two teachers, and wounded nine others, at Apalachee High School in Winder, Georgia. He used an AR-style rifle his father had bought him as a Christmas present.

Sixteen months earlier, an investigator from the sheriff’s office in Jackson County had questioned Colt and his father Colin about an online threat to “shoot up” his middle school. Colt’s mother — whose history with law enforcement included guilty pleas for property damage, trespass and family violence, and warnings about drug possession — no longer lived with them. Colt denied that he was responsible for the threat. Colin Gray revealed that Colt had been bullied at middle school, but insisted his son did not have “unfettered” access to guns. “I have to take you at your word,” the investigator said, “and I hope you’re being honest with me.”

K-12 school shootings occur far more frequently in the U.S. than in any other country. Most shooters give off warning signs before they act. Nonetheless, Republicans in Congress and in many states have often blocked measures to reduce gun violence, including three common-sense initiatives that the majority of Americans support.

First, nine states and........

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