Forensic tests have been unable to establish how a woman thought to have been murdered died, police say.
Officers found the body of the woman in her 60s on the Watlington Hill estate, Oxfordshire, on Friday evening.
A post-mortem examination was carried out on Monday but Thames Valley Police said further specialist tests would need to be carried out and these would "take some time".
A 23-year-old man has been arrested on suspicion of murder.
He was found seriously injured in the National Trust park but remains in hospital in a serious but stable condition.
Officers previously said they did not believe it was a random attack and that it was linking the murder to reports of a man seen acting suspiciously near the Fox and Hounds pub on Christmas Common, near the park, a few hours earlier.
Detectives said the attacker and victim were known to each other.
A formal identification of the victim is still to be carried out, the force said, but her next of kin has been informed.
Scene watches remain in place around the park area.