Terry Marsh was 14 when he gave a video interview to police investigating the murder of his mother Heather Barnett, who was found dead in her flat in Bournemouth, Dorset, in November 2002.
Terry told officers that he and sister Caitlin arrived home just before 4pm.
“My sister started calling out and there was no answer,” he said.
Caitlin then went to the bathroom door and knocked but there was no reply, so she opened it and Terry said she went “absolutely ballistic” when she saw her mother dead on the floor.
“I went into the bathroom. I opened the door halfway. I saw her lying on her back with blood. I saw blood absolutely everywhere and I thought ‘oh no’.
“When I saw the blood I thought this ain’t no accident.”
He told the 999 operator he wanted both the police and ambulance because his “mum’s been murdered”.
Both youngsters went outside and grabbed hold of Danilo Restivo and his then girlfriend Fiamma Marsango who comforted them as the emergency services arrived.
The prosecution allege that Italian national Restivo, murdered Ms Barnett, 48.
The 39-year-old, who lived opposite, denies the charge.
Earlier the jury at Winchester Crown Court heard claims that Restivo has a hair fetish and murdered and mutilated the seamstress with the same “hallmark” method he used to kill a teenager in Italy.
Restivo is alleged to have entered the flat and bludgeoned Ms Barnett with a hammer before cutting her throat.
He then cut off her breasts, while leaving a clump of someone else’s hair in her right hand and some of her own underneath her left hand.
The jury was told the manner in which Ms Barnett was murdered could be considered Restivo’s “hallmark” and linked the killing to the murder of 16-year-old Elisa Claps in Potenza, Italy, in 1993.
Michael Bowes QC, prosecuting, explained that Restivo, who then lived in Potenza, had arranged to meet Elisa Claps at the Most Holy Trinity Church on September 12, 1993.
The teenager was never seen again and her body was discovered in a loft of the church almost 17 years later on March 17 last year.
The case was adjourned until today.