More than a week later, what we know and don’t know about how a gunman carried out a massacre at Robb Elementary School in Uvalde, Tex. — and why the timeline from authorities keeps changing.Read more:In the days since a shooter killed 19 children and two teachers at Robb Elementary School, new and horrifying details about the timeline of events keep emerging. We now know that the gunman was able to walk into the school unimpeded. We know that children called 911 from within classrooms pleading for help. But we still don’t know exactly why it took so long for authorities to stop the gunman. Silvia Foster-Frau reports on what happened during a devastating 90-minute window.