A “mountain of evidence” proves that the 16-year-old boy accused of the abduction, rape and murder of Alesha MacPhail on the Isle of Bute is guilty, a jury has been told.

In his closing address at the high court in Glasgow, advocate depute Iain McSporran QC, prosecuting, said the accused had told “a pack of lies” in the witness box, and dismissed his special defence of blaming Alesha’s father’s girlfriend as “preposterous”.

Alesha, from Airdrie in Lanarkshire, was found dead last July in the grounds of a disused hotel near the seafront home that her grandparents shared with her father, close to Rothesay, where she had arrived a few days earlier for a three-week summer break.

The six-year-old was last seen by her family at about 11pm on 1 July. Her grandmother, Angela King, reported her missing at 6.23am the next day and appealed for help on Facebook to search the island. A member of the public found Alesha’s body about half a mile away at about 9am.

Referring to the accused’s suggestion that, after meeting him for sex on the night of Alesha’s disappearance, 18-year-old Toni McLachlan had taken the used condom in order to plant evidence on the girl’s body after killing her, McSporran asked the jury: “What’s the evidence of Toni McLachlan’s involvement other than the accused’s theory? Not one single piece.”

He went on to list what he described as a “mountain of evidence implicating the accused”. This included fibres believed to be from the accused’s trousers found on Alesha’s clothing, DNA matching the 16-year-old found all over the child’s body and indicating that she had been raped by him, as well as CCTV of a figure on the shoreline who a number of witnesses said appeared to be carrying a heavy object, the timing of which McSporran said “works perfectly” with the other footage, which shows the accused leaving his family home on three occasions during that night.

Describing the accused’s first contact with the police, who were alerted by his mother who grew suspicious having seen his absences recorded on the family’s home CCTV system, McSporran said: “Down comes a confident young man, and spins them a yarn.”

Addressing why there was no scream from Alesha or sound of struggle heard by her family as she was taken from the flat, he suggested this could be explained “if she were threatened by a knife, if her mouth were covered, if she were sound asleep”.

He reminded the jury that a knife matching one missing from a block in the kitchen of the accused’s home had been found discarded on the shoreline near the MacPhails’ flat.

McSporran also told the jury that they must put aside their personal prejudices about people who use cannabis, or people who watch pornography, after they heard previously that Alesha’s father had sold cannabis to the accused, and that he and McLachlan had watched pornography in bed together after the child had gone to sleep.

He acknowledged that the case, which required the jury to hear detailed evidence about the catastrophic injuries that the child sustained to her genitals, “must have been one of the worst [to serve on]”.

Earlier on Wednesday morning, on the eighth day of the murder trial, the court heard from Lindsay Calderwood, a friend of Toni McLachlan, as the defence concluded its case.

Calderwood said that on the morning of Alesha’s disappearance, she and McLachlan were searching together for Alesha, when her friend said “out of the blue” that “she had a bad feeling about [the accused]”.

Asked by Brian McConnachie, for the defence, about her impression of her friend’s relationship with Alesha’s father, Robert MacPhail, she was taken back through the statement she gave to police last July, in which she stated: “I always felt Toni was jealous of Alesha, as she felt threatened by the time and attention he gave to Alesha. She didn’t like it when Rab [Robert] had Alesha staying at the house.”

Asked about her remarks, Calderwood said: “It’s quite normal for stepmums to be jealous.”

The court also heard from Avril Lax, a neighbour of the MacPhail family, who said that she could hear violent arguments between Robert MacPhail and McLachlan through the wall over a three-month period during the summer of 2017.

Asked what she heard, she said: “Usually Toni screaming, the sound of skin hitting skin, Toni shouting: ‘Please somebody help me, please somebody get the police.’” She said that she had not contacted the police but that she had spoken to Robert’s parents, Alesha’s grandparents Angela King and Calum MacPhail. “They told me that I heard nothing. They didn’t want to blacken Robert’s name. There was a court case coming up for custody of Alesha.”

Earlier in the trial McLachlan had denied there was any violence in their relationship. Last week the court learned that Robert MacPhail is currently on bail for a charge of domestic abuse.

The case continues.