Palmer was on bail pending trial in Spain for fraud, money laundering and extortion.
Essex police confirmed on Tuesday that they had launched a murder investigation after his body was discovered at his Brentwood home last week.
It was previously reported that the 64-year-old had died from a heart-related illness but a post-mortem examination confirmed he died from gunshot wounds to the chest.
Described as a serious organized criminal, Palmer was convicted of fraud over a timeshare empire he set up in Tenerife, where Spanish police linked him and associates to swindles, protection rackets and extortion, death threats, violent attacks, as well as money laundering and deals with Russian organised crime.
His name was linked to outbreaks of violence on the streets and in nightclubs in the Playa de las Americas and Los Cristianos resorts.
Palmer was nicknamed Goldfinger after he was accused of handling the gold from the £26million ($40million) Brink's Mat robbery at Heathrow in 1983, one of the greatest crimes of the century.
But he was cleared at a a trial and then moved to the Canary Islands where he made a fortune estimated at €420 million in a timeshare fraud.
He was arrested and jailed in the UK for the fraud in 2001 and served half of his eight-year term reportedly still running his criminal enterprise from behind bars.
But in May, Palmer was arrested in Tenerife and faced 15 years in a Spanish prison accused of fraud, firearm possession and money laundering following a 15-year police investigation.
Palmer had been on bail for eight years after being arrested by armed police in Tenerife in July 2007.
No date had been set for the trial and Palmer was freed on bail and allowed to travel back to the UK where he was shot dead just weeks later.