"Jack, you are my best friend," began the handwritten letter to six-year-old Jack Pinto, one of the 20 first graders and six teachers killed on Friday in one of the worst mass shootings ever at a US school.
"We had fun together. I will miss you. I will talk to you in my prayers. I love you Jack. Love, John."
Under the letter -- written in the shaky handwriting of a child about Jack's age -- were two color photos of the boys together, including one of them stepping out smartly in T-shirts and shorts.
USA Today reporter Yamiche Alcindor came across the letter at a Newtown funeral home on Monday where Jack's friends and family paid final respects before the lad's burial, she told AFP in an email.
Alcindor posted the image on her Twitter account, @Yamiche, on Tuesday, from where it ricocheted across social media.
Police say Adam Lanza, 20, gunned down his victims at Sandy Hook Elementary School with a semi-automatic assault rifle belonging to his mother, who he had shot and killed earlier. Lanza went on to take his own life as police closed in.