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 sexual abusers.

Alexander McCartney from Northern Ireland pretended to be a teenage girl to make friends, then abused and blackmailed children around the world, often sharing images of them with other pedophiles.

Some of the children were four years old. Some had not told anyone about their experiences until the police knocked on their door.

McCartney gradually pleaded guilty to 185 charges including manslaughter After the 12-year-old girl he abused committed suicide.

He was sentenced to at least 20 years in prison.

Following police contact in Scotland, an urgent investigation by the Police Service of Northern Ireland (PSNI) became operational in March 2019.

Detectives located Alexander McCartney’s home address, arrested him, and interviewed him.

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

These devices contained hundreds of thousands of indecent photos and videos of underage girls performing sexual acts while being blackmailed.

McCartney created and used many fake accounts on online platforms, primarily Snapchat, to trap and manipulate them.

PSNI Det Ch Supt Eamonn Corrigan said McCartney was “criminalizing on an industrial scale”.

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

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

In one case, it took McCartney just nine minutes to groom, sexually assault and blackmail a girl who was just 12 years old.

As time went on, it became clear that McCartney’s depravity had spread not only to the UK but to the whole world. The abuse also involved other people, family pets, and objects.

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

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

According to police, he “established a pedophile enterprise” and “stole the childhood” of his victims.

In the spring of 2019, police called Catherine Kierans, the acting head of the Prosecutor’s Office’s Serious Crime Unit.

They said “something big is emerging…involving catfishing.”

Catfishing is when a person creates a false identity to gain people’s trust and exploit them.

Ms Kierans said “little girls, aged approximately 10-12, were being threatened in the most indecent manner”.

He said that some of the abused children disclosed that they had been abused before, while some remained silent.

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

“But some of the children did not tell anyone about their experiences until the police knocked on the door.”

According to Ms Kierans, McCartney was harassing “24 hours a day”.

As the investigation spread around the world, Ms Kierans said prosecutors realized McCartney had been “very meticulous about recording the footage”.

“In some cases, he would also record a map of the child’s whereabouts on Snapchat, which allowed police to locate the child.”

Her 2021 trial was postponed after police discovered that a little girl had committed suicide in the US state of West Virginia.

“The level of abuse from the beginning was so horrific that we feared these children would be OK once they were identified?” Miss Kierans said.

“Unfortunately, our worst fears were somehow realized when we learned that one of the little girls had committed suicide.

“Working closely with American authorities, we were able to prove that this child committed suicide during the abuse, while still online with McCartney.

“At that point, the child’s death was so linked to abuse that we felt we had strong evidence that he had killed her.”

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

McCartney was charged with manslaughter.

Ms Kierans said it was believed to be the first time an abuser anywhere in the world had been held responsible for manslaughter where the victim and perpetrator had never met in person.

The magnitude of the case was so great that prosecutors had to be reasonable about the charges.

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

“All in all, there were around 200 charges (related to around 70 victims) and this is probably one of the largest charges we’ve seen in Northern Ireland.”

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

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

He was just 21 when he first appeared at Newry Magistrates’ Court in July 2019; she had long, fluffy hair and the wide-eyed eyes of someone surprised to sit still.

He was detained in Maghaberry Prison for more than five years; he left only to appear in court and be further questioned by police.

He said little at these hearings other than confirming his name and date of birth and speaking softly to gradually confess his guilt.

McCartney attended Newry High School and was interested in gaming.

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

“Maybe he was on the edge of things, but he had friends who didn’t know anything about it.”

He then took a course at Southern Regional College in Newry, where he was described as “quiet and not very interested in the rest of the class”.

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

The situation was sad for those living in his house and around him.

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

“At first there were whispers, then disbelief. “I’m sure people talk about it in their own homes, but it’s not discussed in public because people don’t know what to say.”

Another said: “He came across as a nice, kind and intelligent young man.

“There’s nothing extraordinary about him.”

But what is extraordinary is the enormity of his crimes; Many of his victims pleaded for the abuse to stop, but prosecutors said he “callously continued by forcing victims to include younger children, sometimes as young as 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 PPS.”

Ms Kierans said despite intense efforts by the police, some victims were still unidentified.

“McCartney’s crimes harmed thousands of children and left them and their families facing traumatic consequences,” he said.

“Their bravery stands in stark contrast to their cowardice in targeting defenseless young girls.”

More information and support for those affected by this story can visit: BBC Action Line.