As previously reported, an independent report was published earlier this week, which suggested that "there were actions that Trust staff could have taken" that might have avoided the tragic incident.
Through the charity 'Hundred Families', which supports families affected by mental health homicides, Alex's mother Jo Greene gave her response to the report's findings this week.
Speaking on behalf of her family, Jo said: “It is nearly six years now since we lost Alex in such a dreadful and traumatic way. There is still not a day goes by when we do not miss him terribly. He was the light of our life, and his loss will stay with us forever.
“We would like to record our sincere thanks to all those NHS workers, paramedics, nurses and doctors who tried so hard to save Alex that fateful day.
“Unfortunately their efforts were not successful but we really appreciate what they tried to do.
“But we remain concerned and angry that Alex’s death was entirely preventable – had the numerous and repeated failings documented in this report not occurred.
“A dangerous, mentally ill man was hurriedly discharged from hospital into the community because of his repeated violent behaviour on the ward.
“He told medical staff he heard voices telling him to “hurt people” and even tried to strangle a doctor.
"He was then knowingly discharged into the care of our young family with children who were not told about his serious violence in hospital. That should not have happened.
“The doctors are still arguing about who actually discharged him, and to this day we have still not received any adequate explanation.
“We know now (my Dad) Stewart Greene was discharged without an adequate care plan, or even sufficient medication to keep him safe and well. That should not have happened.
“When we tried to raise serious concerns we were not listened to. That should not have happened.
“These failings placed our family at serious risk, and, we believe, ultimately cost Alex his life.
“We sincerely hope such similar failings will never be allowed to happen again and that this report may go some way to making real changes to protect any other family
from having to go through what we have experienced.
“I can only hope lessons will truly be learnt from Alex’s death."
Jo Greene also offered a few words in tribute to her son.
“My son Alex was a vibrant, loving, and unique nine-year-old boy. In many ways he was just like most boys his age playing on his Xbox with his friends.
“But he got most of his enjoyment watching his beloved ‘fast trains’.
“As a family we would often take trips to local mainline stations to watch the London to Edinburgh trains whizzing past. He was also a huge fan of the Red Arrows and super cars.
“He was such an inquisitive and bright boy.
“I remember being in the Doctors’ surgery one day and Alex asking the doctor “Why can’t humans be powered by electricity”?
"The doctor was amazed. She told him he would grow up to be a famous scientist with ideas like that.
“Alex was obsessed by wind turbines he would watch them for ages as we sat on the beach, his sister building sand castles.
“And his favourite school trip was a visit to a power station, and his teachers were amazed by some of the questions he asked.
“Alex was so close to his tenth birthday when he was killed, just days before Christmas 2014.
“Today he would have been nearly 16. It is so hard for me to imagine him as a teenager.
“I watch his friends growing up and find I think about what he would look like and what he would like doing now.
“For me though, Alex is always that cheeky, curious and imaginative little boy - the boy who dreamt of being a train driver so he could whizz all over he country.
“The day Alex died, it felt like my life had ended.
“And yet, I had another little person that needed me to be mummy. I wanted to just shut myself away and drift away, but I knew I couldn’t.
"I knew I had to make sure that my daughter had the childhood she deserved. She barely remembers her brother now, and that is heart-breaking, because she idolised him.
“As well as carrying on for Evie, I knew I had to fight for the justice that Alex deserves.
“Alex’s favourite time of year was Christmas. His first passion in life, even before trains, had been lights of any kind.
“From street-lights to disco lights he was fascinated. So, putting the tree up or the local Christmas light switch on were always big events.
“Christmas should be a time of happiness and excitement. But now every December, I am filled with unbelievable sadness and pain.
“I know I have to make Christmas special for my daughter, but also for Alex’s memory.”