Murder squad detectives were called in after the body of an elderly woman was discovered.
Worried neighbours and friends raised the alarm last night after the woman had not been seen for several days.
Police broke into the address in Richmond Court, off Wykeham Road, Worthing, shortly after 11pm and discovered her body.
The woman, in her eighties and who lived alone, had been dead several days and her body had decomposed.
Police found signs of a possible break-in and they are treating the death as suspicious.
A post mortem was being carried out this afternoon to determine how the woman died.
An investigation, codenamed Operation District was launched and detectives and uniform officers were carrying out house-to-house inquiries.
The area was also taped off by police.
Neighbour Muriel Weeden said: "She was a very nice lady and used to love her cats.
"She was a very friendly neighbour and I'm sorry to hear she has died."
Another neighbour said: "She kept herself to herself and we didn't see her much but obviously this is a shock."
A police spokesman said this morning: "We are treating the death as suspicious and a post mortem is to be carried out later today."
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