Cassidy Stay, 15 raised the alarm after a gunman shot her mother, father, two sisters and two brothers in Texas on July 9 this year.
At a memorial for her family members, Stay moved everyone by quoting lines from 'Harry Potter's fictional headmaster Albus Dumbledore: "Happiness can be found even in the darkest times if one only remembers to turn on the light."
Rowling soon contacted her by sending a letter as Dumbledore in purple ink. The author also posted a wand, an acceptance letter to Harry Potter's school Hogwarts, a list of school supplies and a signed book, reported the Telegraph.
"She was also sent a wand, an acceptance letter to Hogwarts with a school supply list, along with the 3rd book with JK's autograph.
A spokesman for the 49-year-old author confirmed that Rowling was in touch with Stay. "However, the contents of the letter remain private," the spokesman said.