Nicole Hockley lost her son Dylan to the worst school massacre in America's history.

The 6-year-old was shot and killed at Sandy Hook Elementary in Connecticut on December 14, 2012. He was one of 24 students and teachers who died when the gunman entered the school and opened fire.

The Hockley family grieved, and is still grieving. Four years after losing her beautiful boy, Ms Hockley has written a heartbreaking letter to the mother of America's latest school shooting victim, Jacob Hall.

Renae Hall lost her son because a 14-year-old entered a classroom and starting shooting at Townville Elementary School in South Carolina last Wednesday. Brave Jacob was shot in the leg and died on Saturday.

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Ms Hockley's letter, shared in full by the New York Daily News, describes her struggle and informs Ms Hall she's not alone. But it starts by declaring she is "ashamed" for thinking nobody had died and for moving on.

"I'm ashamed of myself," she begins.

Renae Hall, right, mother of Jacob Hall, reacts as her son is placed into a hearse during a superhero-themed funeral service at Oakdale Baptist Church. Photo / AP
Renae Hall, right, mother of Jacob Hall, reacts as her son is placed into a hearse during a superhero-themed funeral service at Oakdale Baptist Church. Photo / AP

"When I heard the news of the Townville Elementary school shooting, I was horrified. Too many similarities to the shooting at Sandy Hook School that took the life of my son, 19 of his classmates and six of his educators almost four years ago.

"My first question was, 'Are they OK?' Hearing that there were no deaths at the school eased my pain a little and I was able to return to my work. I knew the community would be traumatised.

"I knew families would be left wondering how this could have happened and what they could do to prevent it from happening elsewhere. I knew eventually we would hear about the signs and signals that the shooter gave off long before the violence occurred, but that were missed, not recognised, or not acted upon. I knew it was yet another preventable tragedy, but my immediate pain and fear was lessened because none of the children had been killed.

"Until Saturday, when I learned of Jacob Hall's death. Seeing the picture his family released before he succumbed to his wounds killed me inside. Seeing his mother's face as she held her beautiful baby boy for the last time ... there are no words for the anguish I felt then and still feel today.

"I also felt deeply ashamed. Ashamed that I had allowed a part of myself to accept the Townville Elementary shooting as another day in America, just because no one had died.

For me to be complacent about gun violence, even for a moment, is unforgivable. I know the price of complacency and inaction. I know that until enough people are actively engaged in preventing firearm deaths and injuries, that change will not come."

Dale Hall, dressed as Superman, stands before the casket during a superhero-themed funeral service for his brother, Jacob Hall. Photo / AP
Dale Hall, dressed as Superman, stands before the casket during a superhero-themed funeral service for his brother, Jacob Hall. Photo / AP

"Renae - I am so sorry that I looked away, even for just a moment. Your son deserves so much more than that, as do you."

She said she saw herself in Ms Hall, and has a "sense of the journey" she is on and the pain she is in.

"I heard you speak about Jacob and thought of the similarities with my Dylan. Both boys were born six weeks early. You had the joy of Jacob in your life for six years and four months, while I had my happiness with Dylan for six years and nine months. Jacob was your superhero; Dylan was my Superman."

Ms Hockley said she was pleased Ms Hall could find forgiveness through her faith, something she has never been able to achieve.

"I'm sad to say I have not found that yet - I am still mad at God."

She offered some advice but knows it will be cold comfort.

"In time, I know you will find your own path and moments of happiness again, eventually even joy, but the absence of Jacob in your life leaves a hole no moments can ever fill.

"As a mother and grieving parent, I am here for you now and always, in any way I am able.

"My heart is with you ... I promise you, and all the other parents and grandparents receiving this message, I will never allow myself to be complacent for a moment again."