Detective Chief Inspector Phil Reade of Greater Manchester Police said: "This is a shocking case in which an innocent and defenceless little boy has suffered appalling injuries at the hands of someone who had been trusted to look after him.
"Rigby has today been put behind bars for a long time for his actions, but this will go little way to ease the suffering of Rio's family, who have lost their beloved little boy under such tragic circumstances."
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Police were called in amid fears the man may have been poisioned.