The search for clues in the body in the bunker mystery is being hampered by the rough terrain at Beacon Hill Fort.
Two teams of officers were still working at the historic site in Harwich on Thursday - scenes of crimes officers were carrying out a detailed search of the bunker where the man's body was found on Sunday evening and another team was carrying out a general search of the entire fort site.
An Essex Police spokeswoman said: "The site has lots of tunnels and bunkers and it is proving difficult terrain to work in as it is so overgrown.
"Officers are having to rope off areas for their own safety but they are making progress."
Police are trawling through details of missing people in a bid to find out the identity of the man whose partially decomposed and mutilated remains were found by two passers-by.
She said: "Officers are looking through the details of missing people, not just in Essex but nationwide."
It is believed the body may have lain dead in the bunker since Thursday or Friday last week and police want to hear from anyone who have had friends or relatives missing since then.
An incident room has now been set up at Stanway police station which is being manned by 12 personnel.
Post mortem examination results were inconclusive but it is known the body was set alight and mutilated after the man died.
As well as the possibility of the victim being murdered, it is also possible he may have died of natural causes and the body was tampered with by someone who stumbled across it.
Anyone with information should contact the incident room on 01206 546629 or Crimestoppers on 0800 555111
The spokeswoman said the main thrust of the investigation is to find out who the man was.
Concerns over security
Concerns over the security of the Beacon Hill Fort site have been raised for years.
As well as being a haunt for people sleeping rough because of its network of tunnels and its isolation, the grounds have also been popular with children, who have been warned about its dangers in the past.
Two years ago fire crews were called to the fort after youngsters set fire to rubbish in the tunnels.
The Harwich Society has been calling for increased security at the fort which was originally built in the 1890s to protect the approach to Harwich Harbour.
Tendring Council owns part of the site along with Essex County Council and a private landowner. A Tendring Council spokesman a working party is meeting this week about the ownership of the site.
With many of the tunnels having just one way in and out, fire officers feared children would get trapped by the flames and smoke.
An early type of radar tower was added in 1941 but the site has not been used since the end of World War Two. Society secretary Andy Rutter said: "It is an on-going situation and only recently a building in nearby Cliff Park was vandalised.
"The problem is that areas of the site are owned by different people which has blocked any progress and that will still carry on until it is sorted out."
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