The emergency services were called to reports of an injured man at 2.41pm on Tuesday, but he was pronounced dead at the scene around twenty minutes later, Scotland Yard said.
The victim was working at the venue at the time. His family have been informed but formal identification has not taken place.
Initial reports suggested the man may have fallen to his death, but police now believe he was the victim of a serious assault.
A 24-year-old man was initially arrested on suspicion of murder on Thursday and was taken to a south London police station.
He was later de-arrested for the murder, then arrested on suspicion of assisting an offender and he remains in custody.
DCI Luke Marks said: “At this early stage in the inquiry, it is believed that the victim was viciously attacked by an unknown suspect, inflicting injuries that have resulted in his death.
“We have made one arrest but our investigation continues apace."
Detectives are hunting a second man who left Hatton Garden and boarded a central line train at Chancery Lane Tube station at around 2.15pm.
He is described as a white male aged in his 20s who was wearing a navy blue fleece jacket, blue jeans and brown boots at the time.
"Were you in the Hatton Garden area at that time and did you see a man running towards Chancery Lane, or otherwise acting suspiciously?" Mr Marks said.
"We would urge you to come forward as soon as possible."