The partner of the murdered criminal John “Goldfinger” Palmer has said she is haunted at the way a suspected hitman stalked him before shooting him six times.

Palmer – once described as Britain’s richest criminal and thought to have been worth £300m – was found dead in the garden of his woodland home in South Weald, Essex, on 24 June 2015.

His family has put up a £100,000 reward, with the support of Crimestoppers, for information leading to the arrest and conviction of his killer or killers.

DCI Stephen Jennings said there was “no reason for us to believe that the money put up by members of John’s family is the proceeds of criminality”.

The spyhole believed to have been scratched by the suspected hitman. Photograph: Essex police/PA

Essex police have released crime scene photos of the hole in the 5ft-high fence panel, believed to have been scratched by the suspected hitman 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 he had recently undergone chest surgery, but it later emerged he had been shot six times at close range.

An inquest in 2016 heard that there were failings on behalf of Essex police and concluded Palmer was unlawfully killed.

Christina Ketley, 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.”

Jennings said he believed it was a “professional contract killing” which happened in the only part of the garden not covered by CCTV. He believed the killer watched Palmer until he went to make a bonfire to burn some old documents, then jumped over the fence.

The spyhole is in the centre of the garden 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, Jennings said.

Five of the six bullets had been found; all were fired by the same weapon.

Palmer was shot in the right elbow, right breast, right upper abdomen, top of the back, left renal area and left bicep; this last bullet has not been recovered.

The ballistics expert concluded that Palmer could initially have been ableto run a short distance until he lost consciousness from blood loss and died.

Palmer’s body was found 50ft-65ft from where he was thought to have first been shot, Jennings added. It is possible that three of the bullets were fired while Palmer was standing and three while he was on the ground.

A bullet recovered from the scene of the shooting

Jennings said Palmer, who gained his nickname after being acquitted of handling gold bullion in the £26m Brink’s-Mat raid in 1983, had associated with some of the people convicted over the Hatton Garden raid in 2015 and had been due to stand trial in Spain over alleged real estate fraud.

“My belief is the person responsible in arranging this is someone very high up in the criminal underworld and we would expect if finances were used to purchase this killing you’re talking about a large sum of money,” he said.

He said he believes the key to solving the case lies within the criminal fraternity in the UK and possibly Spain, and also appealed to the gunman. “Gunmen can be hired as there’s a financial benefit, but they can also be hired on the basis of debts owed or threats,” he said.

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.