Detectives investigating the death of John "Goldfinger" Palmer have arrested a man on suspicion of conspiracy to murder.
The 65-year-old - who was cleared of handling proceeds from the £26 million Brink's-Mat bullion raid at Heathrow in 1983 - was found shot in his home in South Weald, Essex, on June 24.
Essex Police said a 43-year-old man from Rugby was arrested on Wednesday. He has been bailed pending further investigations.
After Mr Palmer's body was found in his house in Sandpit Lane, South Weald, police did not initially treat the death of the notorious criminal as suspicious, blaming complications from a recent operation.
But a post-mortem examination later found he died of gunshot wounds to the chest.
Speaking after the arrest, Detective Chief Inspector Simon Werrett, who is leading the investigation, said: "This is a positive development in the enquiry but we are continuing to appeal for witnesses to any suspicious activity in the area to come forward or for anyone who has information which could help us to get in touch.
"Sandpit Lane is a popular spot for joggers and dog-walkers and I am particularly keen to hear from anyone who was in the vicinity between 4pm and 6pm on Wednesday June 24."
Mr Palmer was known to have extensive connections in the criminal underworld and earned his nickname after being cleared over the Brink's Mat raid.
He was jailed in 2001 for timeshare fraud, after building a multimillion-pound business that cheated thousands of Britons who wanted a holiday home in the sun.
He had spent the last eight years on bail following his arrest in Tenerife in 2007, where he was charged with fraud, firearm possession and money laundering.