Police are "no closer" to solving the murder of notorious criminal John "Goldfinger" Palmer a year-and-a-half after he was shot six times at close range.
Essex Police originally thought he died of natural causes because of recent keyhole surgery to his chest, but it later emerged he had been murdered and that the crime had "all the hallmarks" of a contract killing.
The 65-year-old - once described as Britain's richest criminal and thought to have been worth £300 million - was found in the garden of his remote woodland home in South Weald, Essex, on June 24 last year.
The force confirmed that officers had not properly inspected his body, missing the fact he had been shot at close range, and did not check his criminal history so were oblivious to his high profile.
Two police officers who missed crucial signs that Palmer was murdered had "management action" taken against them.
A post-mortem examination conducted at Basildon Hospital a week later found Palmer had been shot six times and ballistics experts concluded the weapon used was a pistol fitted with a silencer which has never been found, a hearing at Essex Coroner's Court in Chelmsford heard on Tuesday.
A murder investigation was launched after the post-mortem examination.
Detective Chief Inspector Stephen Jennings told the inquest into Palmer's death that there were "failings on behalf of Essex Police".
He said more than 700 lines of inquiry had since been conducted, more than 200 witness statements had been taken and the investigation was ongoing.
But he said this was complicated due to Palmer's "lifestyle and his previous involvement with criminality", and there were a large number of people who could have wanted him dead.
Palmer was sentenced to eight years in prison in 2001 for a £33 million time share fraud which had 16,000 potential victims, which Mr Jennings described as "16,000 motives".
Mr Jennings also said Palmer, who gained his nickname after being acquitted of handling gold bullion in the £26 million Brink's-Mat raid in 1983, had associated with some of the people convicted over the £14 million Hatton Garden raid in 2015.
Palmer was also being investigated by the Spanish authorities and was due to stand trial over alleged real estate fraud involving properties in Tenerife and Spain, with a trial set for 2017.
Mr Jennings said the timing of the killing played a large part in the ongoing investigation, but added: "To be honest, we're no closer to finishing this inquiry."
He said: "We believe it was very much a contract-style killing and the search is not just for the gunman; it's for the person who wanted to cause him harm and the person who may have financed the operation.
"It may have taken weeks, if not months, of planning before it took place."
Mr Jennings said it was thought that a suspect drilled a hole in the wooden fence around Palmer's property and watched him until he went into a far corner to make a bonfire to burn some old documents shortly after 5pm.
He believed the suspect jumped over the fence and shot Palmer, who then stumbled no more than 15 metres and was found by family members who alerted the emergency services.
Nobody has been brought to justice for the killing.
Senior Essex coroner Caroline Beasley-Murray concluded Palmer was unlawfully killed.
Palmer's son-in-law Ashley Thilthorpe attended the hearing at Essex Coroner's Court in Chelmsford and made no comment.