15-year-old Cassidy Stay from Texas, survived the attack by playing dead. JK Rowling wrote to Cassidy to offer comfort.
Cassidy's mother, father, two younger sisters and two younger brothers were murdered by a gunman in an attack on July 9.
The teenager quoted the fictional headmaster, Albus Dumbledore in a memorial service speech she made a few days after the murders.
“Happiness can be found even in the darkest times if one only remembers to turn on the light,” she said, quoting from Harry Potter.
An online campaign to persuade JK Rowling to contact the teenager led to the author writing to Cassidy in the voice of Dumbledore, according to The Telegraph.
The handwritten note of condolence was written in purple ink and accompanied by a wand. The transatlantic package also included an acceptance letter to Harry Potter’s school Hogwarts, a list of school supplies and a signed book.