Police were called by the London Ambulance Service on Tuesday to reports of a man injured on the roof of the Pressman Mastermelts building in Hatton Garden, EC1.
Officers attended and the victim, a 47-year-old builder who was carrying out work at the site, was pronounced dead at the scene.
Initial reports indicated that the man may have fallen from height, but officers now believe he was the victim of a serious assault.
A post-mortem examination took place at St Pancras Mortuary on Thursday and gave cause of death as blunt force trauma to the head.
Work continues to inform all next of kin – formal identification has not taken place.
The Met’s Homicide and Major Crime Command is investigating, led by Detective Chief Inspector Luke Marks.
DCI Marks said: “Our enquiries have established that the victim was viciously attacked by a male suspect who is understood to have worked at the same site.
“We continue to appeal for information to trace a man who made off from Hatton Garden towards Chancery Lane, and boarded a Central Line train at Chancery Lane tube station at around 14:15hrs.
“Were you in the Hatton Garden area at that time, and did you see a man running towards Chancery Lane, or otherwise acting suspiciously? We would urge you to come forward as soon as possible.”
The man is described as a white male, aged in his mid 20s. He was wearing a navy blue fleece jacket, blue jeans and brown workmen’s boots.
If you have any information that could assist the investigation, please call the Incident room on 0208 358 0200 quoting CAD 4481/17Apr. Alternatively, call independent charity Crimestoppers on 0800 555 111 if you would prefer not to give your name.
A 24-year-old man was initially arrested on suspicion of murder on Wednesday, 18 April. He was taken to a south London police station.
He was subsequently de-arrested for the murder offence, and then arrested on suspicion of assisting an offender.
He has been bailed pending further enquiries to a date in mid-May.