Murder victim Mengmei Leng (far right) with her uncle and killer Derek Barrett. Photo / Supplied
It's the horrifying, depraved mystery, found in the kitchen of an elderly woman in a leafy Sydney suburb, and now it could lead to a convicted killer having years piled onto his already lengthy sentence.
Derek Barrett is already serving a 46-year jail term for murdering his niece Mengmei Leng in April 2016 — described as a crime of "extreme brutality and great wickedness".
In 2017, he pleaded guilty to stabbing the Chinese national and student 31 times in the Campsie home he shared with her aunt before dumping her body in a blowhole at Snapper Point on the Central Coast.
However, the 32-year-old is now facing fresh charges after a puzzling discovery of a mysterious USB stick, which contained a video which Barrett made of himself sexually assaulting Leng before her bloody murder.
Barrett on Wednesday faced the NSW Supreme Court where he pleaded guilty to 17 offences, including multiple counts of aggravated sexual assault and indecent assault.
But police and prosecutors admit they are stumped over the discovery of the device that led to the fresh charges.
The court was told that in November 2019, the elderly woman was found in her house in Strathfield holding the USB device in her hand.
When her daughter arrived at her home in Sydney's inner west to check on her welfare, the woman was discovered at her kitchen table holding a heavy-duty, weatherproof USB device.
The woman said she thought it was a toy and could not explain how it came to be in her possession.
Crown prosecutor Huw Baker SC told the court the woman was housebound and only ever ventured to her front yard.
"There is no explanation as to how the USB ended up in the hands of the woman in the house in Strathfield," Baker told the court.
The woman had no discernible connection to Barrett, lived about six kilometres from his former Campsie home, and the USB device was discovered nearly four years after Ms Leng was murdered.
When the daughter plugged the USB into a computer, she saw it contained graphic videos and photos and contacted police.
The USB contained nine video files – which run for over an hour – and 13 picture files of the sexual assault of Leng shortly before her murder on April 22, 2016.
Police were able to match the photos to ones Barrett deleted from his phone before his arrest in an attempt to hide his crimes from police.
In 2017, Barrett was sentenced to 46 years in prison, with a non-parole period of 34 years and six months.
However, he could be facing further time in prison when he is sentenced next year.
In 2017, he pleaded guilty to binding and gagging Leng before murdering her and then driving one hour north and dumping her body off a cliff before it was discovered a short time later.
When sentencing Barrett in 2017, Justice Helen Wilson described him as "depraved and sadistic".
"This was a despicable crime of extreme brutality and great wickedness. Its motivation was sexual," she said.
Barrett was also previously sentenced for filming himself sneaking into Leng's bedroom late at night and masturbating as she slept.
However, authorities were not aware of the video of him sexually assaulting Leng just before her murder and charged him late last year.
"Clearly, the offences have not been dealt with," Justice Wilson said on Wednesday.
Forensic psychiatric Dr Richard Furst told the court Barrett had feigned amnesia as a way to mitigate his responsibility for the crime after claiming he could not remember the period in which he detained and killed Leng.
He also said Barrett had attempted to explain his offending by claiming he was suffering from a methamphetamine addiction.
However Dr Furst said his claim was contradicted by his admission he gained weight during the same period.
"In my 30 years as a criminal lawyer, I have not seen an obese methamphetamine addict," Justice Wilson said on Wednesday.
Dr Furst said Barrett exhibited a lack of insight into his crimes and a lack of empathy for his victim.
"[Taking] images of the victim bound and gagged with a look of horror prior to her death and disposing of the body in a blowhole at Snapper Point, a long distance away, were actions highly suggestive of sexual deviance," Dr Furst told the court.
Barrett will be sentenced by Justice Wilson in February.