ST. LOUIS -- The 19-year-old gunman who forced his way into a St. Louis school and killed two people purchased the AR-15-style rifle from a private seller after an FBI background check stopped him from buying a weapon from a licensed dealer, police said Thursday.An FBI background check “successfully blocked this sale,” police said, though they didn't say why the sale was blocked.Police noted in the release that Missouri does not have a red-flag law aimed at keeping firearms away from people who may be a danger to themselves or others.Police said that when Harris shot at officers, they shot back and broke through the door.The St. Louis shooting was the first school shooting to involve multiple deaths since a gunman killed 19 children and two teachers at Robb Elementary School in Uvalde, Texas, in May, according to a list of shootings compiled by Education Week."