An online predator who led an American girl and her father to take their own lives has been jailed for at least 20 years after the UK’s largest “catfishing” case.

Warning: This article contains references to suicide and child sex abuse which readers may find distressing.

Alexander McCartney, 26, previously admitted 185 charges, including the manslaughter of a girl who took her own life.

Police called him a “dangerous, relentless, cruel paedophile” who “may as well have pulled the trigger himself” – and said there were about 3,500 victims.

Devices seized from his bedroom contained hundreds of thousands of indecent photographs and videos of underage girls.

Belfast Crown Court heard they were aged between 10 and 16 and based in the UK, USA, continental Europe, Australia and New Zealand.

McCartney, from south Armagh in Northern Ireland, used Snapchat and other sites to pose as someone else online (known as catfishing).

He pretended to be a young girl to persuade his victims to send him images. He then blackmailed them into sending more explicit material.

Cimarron Thomas, 12, from West Virginia, shot herself with her father’s gun, instead of complying with his demands after he gave her an online countdown. But there were more tragic repercussions.

Unable to live with the loss, her father Ben Thomas, a former US Army veteran, died by suicide 18 months later.

Former computer science student McCartney pleaded guilty to one count of manslaughter, 59 of blackmail, and 70 of inciting a child to engage in sexual activity.

At his sentencing, the judge said his offending began at age 14.

‘Utterly remorseless’

McCartney admitted dozens of charges related to making and distribution of indecent images of children and appeared “utterly remorseless”, said barrister David McDowell KC.

The prosecution said he deliberately targeted victims who were either gay or exploring their sexuality and “degraded and humiliated them”.

The judge called the details “excruciating” and much of what he did is too graphic and disturbing to detail.

McCartney told one girl he would send people to rape her if she didn’t comply. In some instances, he demanded his victims involve younger siblings.

Girl shot herself after McCartney countdown

In May 2018 and posing as ‘Sarah’, he messaged Cimarron Thomas at her family’s farmhouse in the tiny US village of Bruceton and persuaded her to send a topless photograph.

When McCartney revealed he wasn’t Sarah, he demanded more explicit pictures and threatened to send them to her father.

When she pleaded for him to stop, he told her to “dry her eyes” and involve her nine-year-old sister in the sexually explicit material via webcam.

Belfast Crown Court heard a police officer who reviewed the material said Cimarron was “utterly distraught and sobbing”.

The court heard McCartney “counted down” online from 20 to zero – insisting on more pictures.

Cimarron refused and shot herself.

McCartney’s cynical last message read: “Goodbye and good luck.”

Father couldn’t forgive himself

Cimarron’s younger sister heard what she thought was a balloon popping but found her lying on her parents’ bedroom floor. She died in hospital later that night.

Her father Ben couldn’t forgive himself for leaving his handgun within reach and took his own life 18 months later.

A statement read in court from his wife, Stephanie, said he felt guilty about his daughter’s death and “lost interest in life”.

Cimarron’s family couldn’t understand why she killed herself until three years later when police found the online chat on McCartney’s computer.

In a statement, Cimarron’s grandparents said: “We all have been devastated by our granddaughter’s passing.

“We know that nothing that we do or say will bring her back. But if we can help another family to not have to go through what we did, something good could come out of her death.

“Parents, please keep the doors of communication open concerning the evil of some people online.”

Anyone feeling emotionally distressed or suicidal can call Samaritans for help on 116 123 or email jo@samaritans.org in the UK. In the US, call the Samaritans branch in your area or 1 (800) 273-TALK.