Surrey Police said the matter has now been formally passed to the coroner and an inquest will be held after finding no evidence of foul play. This was despite two post-mortem examinations carried out in November failing to establish Alexander Perepilichnyy's cause of death.
Toxicology samples were also taken from both post mortems but the results have not been released.
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The mayor said a post-Brexit visa system was needed to maintain the supply of skilled migrant workers for London's economy.
Peter Barnett made hundreds of trips from his home in Oxfordshire to Marylebone station without paying the full fare.
Police say he may have attacked more children and urged them to come forward.