It was a phone call from a 13-year-old girl in Scotland in 2019 that eventually led to the capture of a social media predator described as one of the world’s most prolific child sex abusers.

Alexander McCartney from Northern Ireland pretended to be a teenage girl to befriend, then abuse and blackmail children around the world, often sharing images with other paedophiles.

Some of the children were as young as four. Some had never told anyone what they had been through – until police knocked on their door.

McCartney gradually admitted 185 charges including manslaughter after a 12-year-old girl he was abusing took her own life.

He has been jailed for a minimum of 20 years.

Following contact from police in Scotland, an urgent investigation by the Police Service of Northern Ireland (PSNI) swung into operation in March 2019.

Detectives identified the home address of Alexander McCartney, arrested and interviewed him.

Sixty-four of McCartney’s devices were seized at his home in the rural Lissummon Road area outside Newry in four separate raids.

Those devices held hundreds of thousands of indecent photos and videos of underage girls performing sexual acts whilst being blackmailed.

McCartney made and used many fake accounts on online platforms, mainly Snapchat, to entrap and manipulate them.

PSNI Det Ch Supt Eamonn Corrigan said McCartney had been “offending on an industrial scale”.

He groomed victims into thinking they were talking online to a girl of a similar age, before encouraging them to send indecent images or engage in sexual activity via webcam or a mobile phone.

McCartney used the same pattern every time, the detective said, adding: “He threatened to share these images online for the pleasure of other paedophiles and use them to further abuse and harass the already terrified and exploited children.”

In one incident, it took McCartney just nine minutes to groom, sexually abuse and blackmail a girl of only 12 years of age.

As time went on, it became clear that McCartney’s depravity spanned not just across the UK, but across the world. The abuse included involving other people, family pets and objects.

The PSNI worked with colleagues in the United States Department of Homeland Security, the Public Prosecution Service and National Crime Agency, and victims were located in America, New Zealand and at least 28 other countries.

Many of these children were only identified through the evidence detectives located on McCartney’s devices.

According to the police, he “built a paedophile enterprise” and had “stolen childhoods” of his victims.

In the spring of 2019, police called Catherine Kierans, acting head of the Public Prosecution Service’s serious crime unit.

They said something “big was unfolding… it involved catfishing”.

Catfishing is where a person creates a false identity to gain the trust of people and exploit them.

Ms Kierans said little girls “an average age of 10-12 years old [were] being threatened in the most depraved way.”

She said some of the children who had been exploited had previously opened up about their abuse, others had remained silent.

“Some of the children had raised the alarm, which helped police to actually identify him in the first place.

“But some of the children, until police knocked the door, they had never told anyone what they’d been through.”

According to Ms Kierans, McCartney offended “around the clock”.

As the investigation spread across the globe, Ms Kierans said prosecutors realised McCartney had been “very assiduous about saving the images”.

“He would also save the map on Snapchat of where the child was in some cases, and that then enabled police to locate the children.”

His arraignment in 2021 was delayed as police discovered the suicide of a little girl in West Virginia, USA.

“From the beginning, the level of abuse was so horrific that we were fearful that when these children were identified, would they be okay?” Ms Kierans said.

“Unfortunately, our worst fears were realised when we discovered, some way in, that one of the little girls had taken her own life.

“Working closely with the American authorities, we were able to prove that this child took her own life during the abuse, when she was still online with McCartney.

“At that point, the death of the child was so intrinsically linked to the abuse that we felt we had a strong case to say that he killed her.”

That little girl was 12-year-old Cimarron Thomas who, in 2018, shot herself while McCartney was abusing her.

McCartney was charged with her manslaughter.

Ms Kierans said it is believed to be the first time an abuser anywhere in the world has been held accountable for manslaughter where the victim and perpetrator have never met in person.

Such was the magnitude of the case that prosecutors had to be judicious with the charges.

“We couldn’t put 3,000 charges on the indictment,” Ms Kierans said.

“In the end, there were about 200 charges [relating to around 70 victims] which is probably one of the largest indictments that we’ve seen in Northern Ireland.”

McCartney grew up five miles outside of Newry and just off the main road to Armagh city.

It’s about as rural as it gets. Farms, a church and a few businesses.

When he first appeared at Newry Magistrate’s Court in July 2019 he was just 21, with long, fuzzy hair and the wide-eyed look of someone surprised to be sitting where he was.

He has spent more than five years on remand at Maghaberry Prison – leaving only for court appearances and further questioning by the police.

In those hearings, he said little other than to confirm his name and date of birth and to gradually enter softly-spoken guilty pleas.

McCartney attended Newry High School and was into gaming.

One source told BBC News NI: “He was introverted and socially awkward. He didn’t interact with people much outside of his group of friends.

“He was maybe at the edges of things, but he had friends who obviously knew nothing about this.”

He then took a course at the Southern Regional College in Newry where he was described as “quiet and didn’t really get involved with the rest of the class”.

When he was eventually charged in 2019 he was a computer science student at Ulster University.

For those living in and around his home, the case has been harrowing.

“The whole place was stunned,” one resident said.

“It was whispers at the start, then disbelief. I’m sure people talk about it in their own homes but it doesn’t get discussed publicly as people don’t know what to say.”

Another said: “He came across as a pleasant, affable, intelligent young man.

“There is nothing extraordinary about him.”

But what is extraordinary is the enormity of his offending; many of his victims had pleaded for the abuse to stop but prosecutors said he “callously continued, at times forcing the victims to involve younger children, some aged just four”.

According to Catherine Kierans, McCartney’s depravity was such that it was “one of the most distressing and prolific cases of child sexual abuse we have ever seen in the PPS”.

Ms Kierans said some of the victims have still never been identified despite exhaustive efforts by police.

“McCartney’s crimes have harmed thousands of children and left them and their families dealing with the traumatic aftermath,” she said.

“Their courage stands in stark contrast to his cowardice in targeting vulnerable young girls.”

Further information and support for those affected by this story can visit the BBC Action Line.