Published Wednesday, 22 February 2012
Gordon Gallagher was nine years old and was playing with his brother in the garden of their Creggan home in February 1973 when he triggered a booby trap device.
His mother Pat was in the kitchen at the time as the blast blew in the windows.
"I saw all the smoke so I ran out and I was shouting to him," she told UTV. "I tried to lift Gordon, and I started shouting that I couldn't lift him.
"It was the last I saw of him."
Gordon's father was at Mass nearby and heard the explosion.
He saw his son at Altnagelvin Hospital.
Pat Gallagher said: "I could see his wee body was jumping up and down. He spoke and said 'Daddy could you get me a drink?' I looked at the nurse and she shook her head.
"I went out to phone home and said he was conscious and he was even speaking to me. He was wheeled past me when I was on the phone. It was the last I saw of him."
The Provisional IRA told the family their bomb had been harmless because it had no detonator.
It tried to suggest the army had come into the garden and put a detonator in the device.
But the Gallaghers dismiss this version as a lie - they believe senior republicans including Martin McGuinness must know who was responsible for planting the bomb.
One occasion, when Billy Gallagher was tending his son's grave, he spotted Mr McGuinness nearby and challenged him.
"I said Martin, look, it was your crowd that put Gordon in there," Mr Gallagher told UTV.
"He said Billy, poor Gordon, but I was in jail at that time."
Sinn Féin said Martin McGuinness was in jail at the time of Gordon's death and they say he has no information about the incident.
The Gallagher family hope that their call for the truth will prompt former IRA members to come forward and provide them with the full information about exactly what happened 40 years ago.