A “relentless and cruel” online predator who blackmailed girls around the world was sentenced to at least 20 years in prison on Friday after being convicted of the manslaughter of a victim who took her own life.

Prosecutors say 26-year-old Alexander McCartney, from Newry in Northern Ireland, pretended to be a teenage girl and carried out catfishing attacks on 3,500 female victims he contacted on platforms including Snapchat.

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Catfishing is using a fake online identity to deceive victims. Authorities said McCartney encouraged his victims to send intimate photos or engage in sex acts, then extorted them by threatening to share the images.

A 12-year-old in West Virginia killed herself in May 2018 during an online chat with McCartney as he demanded sex acts. The victim’s father died by suicide 18 months later.

Prosecutors said they believe this is the first time someone has been convicted of manslaughter in a case where victim and perpetrator never met.

“Working closely with police, the prosecution team was able to establish to the criminal standard that McCartney’s actions had caused the girl’s death, and he had a case to answer for manslaughter,” said Catherine Kieran, acting Head of the Northern Ireland Public Prosecution Service Serious Crime Unit.

Police have identified victims in 30 countries, including Australia, New Zealand and the United States.

McCartney, who carried out his crimes from the bedroom of his childhood home, pleaded guilty to 185 charges involving 70 children, including the manslaughter charge. At Belfast Crown Court, judge John O’Hara sentenced him to life with no chance of parole for 20 years.

“To my knowledge there has not been a case such as the present where a defendant has used social media on an industrial scale to inflict such terrible and catastrophic damage,” the judge said.

Detective Chief Superintendent Eamonn Corrigan of the Police Service of Northern Ireland said outside court that McCartney was a “relentless and cruel” pedophile.

“We cannot underestimate the devastation that he has caused and the childhoods he has stolen,” the officer said. “There was nothing that was going to stop him, apart from putting him in jail.”