Coroner Fiona Wilcox ruled that the spy would not have been able to lock himself in the bag and was therefore likely to have died at somebody else's hands.
She concluded: 'The cause of his death was unnatural and likely to have been criminally mediated. I am therefore satisfied that on the balance of probabilities that Gareth was killed unlawfully.'
Police have also been unable to explain why his DNA was not on the lock on the bag and his prints were not found on the rim of the bath.
Scotland Yard accepted that its investigation had been flawed from the outset.
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