Wicked Campbell, 19, who murdered and raped six-year-old Alesha MacPhail, is caged in Polmont, near Falkirk.
But despite his sentence of at least 27 years behind bars, Campbell has become pen pals with a sex offender in an English prison – who cannot be named.
An insider told The Sun they write to each other “almost daily”.
It has sparked fury from Alesha’s great-grandad, who said her mum would be sickened at the duo’s letters.
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George Lochrane, 78, told the outlet: “It absolutely sickens and disgusts me to hear of it.”
He went on: “It doesn’t surprise me in the slightest – he’s a beast and a leopard doesn’t change its spots, does it.
“I’m just glad to hear the prison knows about it.”
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Saying the prison should be “ripping up” the letters, he added: “That evil monster shouldn’t be allowed any pen pals. He should be suffering in his own silence and rotting in his cell.”
Guards at the young offender’s centre where Campbell is housed are checking the pair’s daily letters.
Prison staff raised concerns about the correspondence and passed the information over to cops.
A source said: “Campbell and this guy are like best pals.”
They went on: “As soon as it became clear the guy down south was a sex offender the Met got involved to monitor their communications.”
Campbell, who was 16 when he brutally attacked Alesha and left her naked body in a derelict hotel with 117 injuries, is said to be a loner in prison.
He is shunned by others in the jail and is a social recluse.
The Scottish Prison Service said: “We do not comment on individual prisoners.”