As a child Graham Dwyer attempted to kill a younger boy who was seriously injured when Dwyer pushed him off a roof while supposedly playing.
Elaine O'Hara's killer later attacked his victim a second time in a violent frenzy while the child was recuperating from multiple fractures.
Dwyer's first known victim grew up terrified of him and only revealed what happened after the architect was charged in 2013 with the childcare worker's murder.
The revelations are contained in a new book by veteran crime writer Paul Williams which is being serialised today in the Sunday Independent and in a Special Report on independent.ie.
The Almost Perfect Murder examines the extraordinary Garda investigation which unmasked Dwyer and brought him to justice.
Williams reveals the belief among detectives that Dwyer filmed Elaine O'Hara's murder when he lured her to Killakee Woods in the Dublin Mountains on August 22, 2012.
Officers, who were in consultation with a trained garda psychologist, later searched behind electrical fittings and wall cavities in Dwyer's home in the belief that a memory card containing the horrific material may have been hidden there.
Detectives still believe it exists and say they will continue to search.
The assumption is based on an analysis of a sordid story entitled 'Killing Darci' which was found along with other disturbing material that proved Dwyer's fascination with knives, blood, rape and murder.
The shocking material, which had been deleted, was recovered by garda computer experts on a hard drive seized during a search of Dwyer's Foxrock home.
It included videos of the 42-year-old married father-of-three stabbing women, including his victim, while having sex.
In the 'Killing Darci' story Dwyer fantasised about stabbing an American woman to death while having sex.
He then described recording the murder and storing the memory card away in an "old tin box" and putting it in a "very special place" to be watched later. The investigation team at Blackrock later identified the woman featured in the 'story' as Darci Day, a young woman from Maine, USA, who encountered Dwyer on an internet site on which people discussed suicide.
Ms Day gave evidence via video link saying: "His fantasy was basically wanting to stab a woman to death during sex."
Williams interviewed former close friends and colleagues of Dwyer and the crime writer reveals that a man who knew Dwyer growing up had confided to a small number of friends that the killer deliberately pushed him off a building when they were children. Gardai also learned about the incident after Dwyer's arraignment.
A friend of the victim said: "The one thing he remembers is the look in Graham's eyes just before he pushed him. He said his eyes were dead and he knew that, for no apparent reason, Graham wanted to kill him."