Cassidy Stay, 15, was the sole survivor of a gun attack in which her parents and four siblings were killed, in Texas on July 9.
Days later, Cassidy made a moving speech at a memorial event for her family, in which she quoted a phrase said by Hogwarts headmaster Albus Dumbledore in the film based on Rowling's novel Harry Potter and the Prisoner of Azkaban.
"Happiness can be found in the darkest times, if only one remembers to turn on the light," she recited.
Following Cassidy's speech, her supporters started a social media campaign to ask JK Rowling to arrange a personal meeting with the teenager.
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According to the Telegraph, Rowling was so moved after seeing Cassidy on global news sites that she tracked down her contact details and sent her a handwritten letter in the style of Dumbledore.
The letter, written in purple ink, was accompanied by a wand, a letter of acceptance to Hogwarts, a list of school supplies and a signed copy of the third Harry Potter book.
A spokesman for the 49-year-old author said: "We can confirm that JK Rowling was in touch with Cassidy Stay, however, the contents of the letter remain private."
Asked whether a meeting between the two is on the cards, she added: "We wouldn't comment on that but [Cassidy's] is a remarkable story."
The manager of a Facebook page created as part of the social media campaign shared the following post yesterday (August 5).
"This afternoon, I talked to a friend of Cassidy's who confirmed that JK Rowling did, in fact, write her a personalised letter from 'Dumbledore' (hand-written with purple ink).
"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.
"I'm so excited and ecstatic that we were all able to make a difference! How wonderful."
Cassidy's father Stephen, 39, her mother Katie, 34, and her siblings, Bryan, 14, Emily, nine, Rebecca, seven, and Zach, four, were all killed on July 9, when a armed man entered their home.
Cassidy survived by playing dead after a bullet hit her finger and grazed her skull.
When the gunman left the house, she alerted the police that he was on route to kill her grandparents too.
Police caught him before he arrived.
Ronald Lee Haskell, 33, who was formerly married to Cassidy's aunt (her mother's sister) has been charged with capital murder.