The mother of a teenager on trial for the murder of Alesha MacPhail has said her son was “adamant” he had nothing to do with the six-year-old’s death when she questioned him about why he repeatedly left the family home on the night the child disappeared.

The 16-year-old accused, who cannot be identified for legal reasons, denies abducting, raping and murdering the child on the Isle of Bute last July.

Alesha, from Airdrie in Lanarkshire, was found dead in the grounds of a disused hotel near her grandparents’ seafront home, close to Rothesay, where she had arrived a few days previously to spend a three-week summer break.

She was last seen by her family at about 11pm on Sunday 1 July. Her grandmother Angela King reported the child missing at 6.25am on Monday morning and appealed for help on Facebook. A member of the public found Alesha’s body approximately half a mile away at about 9am.

The accused’s mother, who cannot be named, said she had initially helped in the search and, after Alesha’s body was found, reviewed footage from a home CCTV system she had installed when her own mother, who had dementia, was living with her.

She said she had told police the footage showed her son, who had been drinking with friends at home on Sunday evening, leaving the property twice in the early hours of Monday 2 July.

As the footage was played for the jury, the mother identified a figure leaving the house at 1.54am on the morning of 2 July – wearing black trainers, jogging bottoms and a dark fleece top – as her son. She told the court her family lived approximately seven minutes’ walk from the flat where Alesha was staying. The film then showed him returning at 3.35am, no longer wearing the fleece top.

She said she then heard her son take a shower.

At 3.45am, the footage shows the teenager leaving the property again, wearing only shorts, and returning in less than 10 minutes.

She was then shown additional footage of her son leaving the house for a third time, at 3.58am, and returning at 4.07am. She said she had not been aware of this, but again identified the figure as her son.

Asked by the prosecutor, Iain McSporran QC, if she had tried to find out why her son had left the house in the middle of the night, she said he had told her he was looking for his lost phone, and later explained that he had been trying to buy cannabis.

She added: “He was adamant that he had nothing to do with [the girl’s death]. I was happy with that explanation.”

The mother was also shown a photograph of a knife recovered by police from the beach, which she identified as “very similar” to one from a set in her kitchen. When told by McSporran that the police had found a five-knife block with one knife missing in her kitchen, she said she had “no idea whatsoever” how the implement had come to be on the shore.

Prosecutors allege the accused was armed with a knife when he took Alesha from her bed.

The teenager’s mother also identified a black Nike top, jogging bottoms and boxer shorts as belonging to her son – which were similarly recovered from the beach. The accused has been charged with attempting to hide evidence.

Earlier on Thursday, the pathologist who examined Alesha’s body told the court the girl was smothered to death after sustaining catastrophic injuries.

Dr John Williams, who examined the child’s body where it was found in woodland and later carried out a postmortem, told the court she had died “as the result of significant forceful pressure to her neck and face”, and went on to describe “severe” and “extensive” injuries to the child’s genitalia, at least some of which he concluded were inflicted while she was still alive.

He said in his wide experience of carrying out post mortem examinations he had never encountered such catastrophic injuries.

Williams noted the the soles of Alesha’s feet were clean and uninjured. He agreed that this suggested Alesha was carried with her feet uncovered to the place where she died.

The trial continues.