Ava Olsen was on the playground with other first-graders during a 2016 school shooting in Townville, S.C., that took the life of Jacob Hall, 6. She is now home-schooled because of her PTSD. (Ricky Carioti/The Washington Post)

The manila envelope arrived on Mary Olsen's doorstep the day after Christmas, and in its top left corner were three words that stopped her: "THE WHITE HOUSE."

It was addressed to her daughter, Ava, but Olsen opened it and scanned the letter first to make sure the message didn't include anything that might trigger her second-grader's debilitating anxiety. Olsen then called her little girl into their living room, where they sat together on the couch. She gave Ava the note, which, at the top, included the presidential seal.