The mysterious deaths of three elderly people after a lunch where mushrooms were served have gripped Australia.

The mysterious deaths of the three have gripped Australia, with interest only increasing after police on Wednesday searched a local refuse centre for a dehydrator believed to have been used in the preparation of the deadly lunch.

On Sunday, police said they had questioned a 48-year-old woman who served the lunch before releasing her without charge.

The woman had been hosting her in-laws, Gail and Don Patterson. Both aged 70, the couple died in hospital following the lunch on July 29 in Leongatha, a small rural town about 135km (84 miles) southeast of Melbourne, police and local media said on Thursday.

Also at the lunch were Gail Patterson’s sister Heather Wilkinson, 66, who died, and husband Ian Wilkinson, 68, a Baptist pastor who remains hospitalised this week in a critical condition.

The woman who cooked the meal was separated from her husband but police had been told their relationship was amicable. Her children were also at home during the lunch but did not eat the same meal, police said.

“What happened is devastating and I’m grieving too,” the woman told Australian media in a tearful interview outside her home.

Detectives searched the woman’s home on Saturday and took several items. The Sydney Morning Herald newspaper reported that police were also conducting forensic tests on a food dehydrator they had found at a nearby landfill to see if it was linked to the case.

Victoria Police Detective Inspector Dean Thomas said it was not clear what type of mushrooms the guests had eaten, but their symptoms were consistent with those from a death cap, a particularly deadly variety.

The investigation is likely to be lengthy due to the complexity of the case, Thomas said on Monday.

“We presume at this stage it was mushrooms but it’s a complex investigation that I think will take some time,” he said.

The Patterson and Wilkinson families said in a statement in a local newspaper on Sunday that they were grateful “to our wider communities for their outpouring of love, support, and prayers”.

“This support extends not only to those we have tragically lost but also to our family member who continues to bravely fight for his life in the hospital,” the families said.

Deaths from consuming mushrooms are relatively rare in Australia, which has several species including the “death cap” mushroom that are dangerous enough to poison and kill a human.

The three deaths have led to warnings from health experts across the country not to forage for wild mushrooms and to buy from reputable sellers.