Gareth Williams' body was found in a flat here under mysterious condition, leading to intense speculation on the circumstances surrounding the 'spy in a bag' case.
A coroner had ruled last year Williams was unlawfully killed.
However, new evidence review by the Metropolitan Police found that "it was more probable" no other person was present when Williams died in his flat.
At a briefing here today, the police announced the conclusion of its three-year investigation into the incident.
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He said there was no evidence that the MI6 officer had intended to take his own life or that his death was connected to his work.
He said: "Three years of extensive investigative activity have developed a very clear profile of Gareth. He was, without doubt, a private person who was very close to his family and had few other close friends.
"There is no evidence of any animosity towards Gareth, and it has not been possible to identify anyone with a motive for causing him harm."
Police discovered his body inside a zipped-up red sports holdall, in the empty bath of his bathroom.
Despite an intense investigation, no one has been arrested and the circumstances surrounding the death have remained a mystery.
During a seven-day inquest in May 2012, coroner Fiona Wilcox concluded that "on the balance of probabilities", Williams was unlawfully killed.
The dead spy's family said they stood by the coroner's findings.
In a statement, they said: "We are naturally disappointed that it is still not possible to state with certainty how Gareth died and the fact that the circumstances of his death are still unknown adds to our grief.
The renewed 18-month investigation was launched soon after the inquest and involved re-interview of MI6 officers and taking their DNA samples.