Monaghan first hit the headlines when he was arrested as a suspect for the supermarket shooting.
At the time Carroll, 29, had been responsible for a series of so-called "alien abductions" across central Scotland.
The kidnappings were described in such a way as the victims, who were tortured and robbed, told police they couldn't remember anything about their ordeal.
On 13 January 2010, Carroll attended a lunchtime business meeting at Asda in Robroyston, Glasgow.
The enforcer met drug pusher Stephen Glen, who was linked to the Lyons family.
A trial later heard Carroll told him: "You're working for me now. Anybody that doesn't fall into line is going to get banged."
But minutes after delivering the ultimatum, he was sitting in the back of a black Audi A3 when a speeding Volkswagen Golf screeched to a halt in front of the vehicle.
Carroll's two associates fled leaving him trapped in the back of the three-door car.
Two masked men emerged from the Golf and opened fire, shattering the rear passenger windows.
Carroll was shot in the head and chest - 13 times in total - in an attack that lasted 25 seconds.
Despite being arguably the most public gangland hit ever carried out in Scotland the Police Scotland investigation was initially met with a wall of silence.
But in August 2010 Monaghan was arrested, just 10 days before his partner gave birth to their daughter.
He spent six months on remand only to be acquitted of the murder in May 2012 after a judge ruled that there was insufficient evidence to convict him.
At the time Monaghan said: "It's been a nightmare. I'm glad to get this over.
"I've always said it was nothing to do with me."
Less than five years later Monaghan was shot in the shoulder outside a Glasgow primary school after dropping his child off.
The gunman was pushing a child's buggy when he opened fire on Muirdykes Road near St George's Primary, Penilee.
Two associates of the Daniel clan were both cleared of the attack at a trial but were later convicted for other organised crime offences.
Monaghan is believed to have moved to Spain soon after the school shooting.
It has been reported that he owned the Costa del Sol bar, bearing his name, where he was killed.
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