The family of notorious murdered criminal John "Goldfinger" Palmer have offered a £100,000 reward in an attempt to catch whoever was responsible for his death.
Palmer, who was once described as Britain's richest criminal and thought to be worth £300m, was shot six times in the garden of his woodland home in South Weald, Essex, in June 2015.
Detective Chief Inspector Stephen Jennings said there is "no reason for us to believe that the money put up by members of John's family is the proceeds of criminality".
Essex Police have released crime scene photos of the hole in the 5ft (1.5m) tall fence panel, believed to have been scratched with a nail or another sharp object, and of a bullet recovered from the scene.
Officers originally thought Palmer had died of natural causes because of recent keyhole surgery to his chest, but it later emerged he had been shot six times at close range.
An inquest in 2016 heard there were "failings on behalf of Essex Police" and concluded Palmer was unlawfully killed.
Christina Ketley, Mr Palmer's partner, said: "It haunts us every day to think that whoever was responsible was clearly watching John, stalking him like an animal before so brutally and callously ending his life.
"Losing a loved one is hard enough but in this case we have the added stress of not only not knowing who did it but why."
Mr Jennings said he believed it was a "professional contract killing" which happened in the only part of the garden not covered by CCTV.
He said he believed the killer watched Palmer until he went to make a bonfire to burn some old documents, and then jumped over the fence.
Palmer was last seen on CCTV at 5.18pm and his body was found by family members, who called the ambulance service at 5.30pm.
A ballistics expert concluded that the gun used was a .32 calibre silenced revolver, which has never been found, Mr Jennings said.
He added that five of the six bullets had been found, but the sixth is outstanding despite extensive searches.
All bullets recovered were fired by the same weapon, he said.
Palmer was shot in the right elbow, right breast, right upper abdomen, top of the back, left renal area and left bicep - this latter bullet has not been recovered.
The ballistics expert concluded that Palmer could have been conscious, initially able to walk, talk, shout and possibly run a short distance until he lost consciousness from blood loss and died.
Mr Jennings said he believed Palmer may have seen his killer.
Detectives are continuing to appeal for information about a man and woman seen near Palmer's home the day before the murder.
A man was also seen in Weald Country Park, next to Palmer's home, at 5.50pm on the day he was shot.
He was white, in his early 20s, around 5ft 10in and of slim build, with short dark blond hair.
He was wearing light blue jeans and a light-coloured baggy sweat top.
Anyone who believes they have information about the murder of Palmer is asked to call Essex Police on 101 or Crimestoppers anonymously on 0800 555 111.