A shopkeeper found dead in his store may have been the victim of a ritualistic killing, police said.
Mr Sivakumar was married with three children
Subramaniam Sivakumar, 44, was found laid out on the ground, hands by his side, with a shoelace around his neck and bags of rice placed over his body.
Before his death on 5 January, a group called the Tiger Boys had visited him at his shop in Willesden, north-west London, although no violence was used.
A fresh appeal for information will be made on BBC's Crimewatch on Wednesday.
A £20,000 reward is being offered for information leading to the arrest and prosecution of those responsible for the murder.
Initial tests failed to establish a cause of death but further examinations found Mr Sivakumar died of a heart attack due to heart disease.
There were signs on his body that he had been restrained and there were puncture wounds behind his ear, said police.
The Tiger Boys became part of the inquiry after detectives found an entry in Mr Sivakumar's diary dated 13 October 2005.
It read: "Today I agreed to give 15,000 to Tiger Boys".
Det Insp Tim Wilkinson said: "He was approached by the same people shortly before his death."
Officers would like to hear from anyone involved with the group or anyone who can shed any light on their identity.
They are appealing in particular to the Tamil community in helping them solve the case.
Mr Sivakumar, who was married with three children, was a Tamil from Sri Lanka.
Police want to speak to Mr Sivakumar's former employee
Mr Wilkinson said he was keeping an open mind about how and why Mr Sivakumar was killed.
"It's a real mystery and our investigation has uncovered a number of issues which we're throwing open to the public so we can sort the wheat from the chaff," he said.
Officers are keen to contact a former employee of Mr Sivakumar's store, Apna Bazaar, on Willesden High Road.
Jeyanthan Anandarrajah, aged 20, also known as Apan, was the last person to see Mr Sivakumar before his death.
After co-operating with the inquiry, he returned to his family in Germany.
But he has since gone missing and police, who consider him a significant witness, say they are concerned for his safety.