Toxic gas is being pursued as a primary line of inquiry after two people and a dog were found dead on a boat on the Norfolk Broads.
Investigators have ruled out suspicious circumstances surrounding the deaths of the man in his 60s and woman in her 50s.
It is thought they may have succumbed to carbon monoxide poisoning.
The bodies were discovered on the small, white motor cruiser shortly after 4pm on Thursday.
The alarm was raised because of concerns over the length of time the vessel had been moored in one spot on the River Bure next to Wroxham Island.
The boat, which is privately owned, has since been towed away to a nearby boatyard where officers are continuing to work with the Police Marine Investigation Branch to assess the vessel.
A police spokesman said: 'The deaths are being treated as unexplained and post-mortem examinations are due to take place next week.
'However, officers do not believe there to be any suspicious circumstances.'
The police are not releasing any more details about the victims until they have been formally identified and their families have been informed.
Boats used on the Broads are often equipped with an electric fridge and a gas stove. This vessel may have been an American petrol-driven model.
When they arrived at the scene, emergency services first thought they were dealing with a 'hazardous material incident'.
However it is believed investigators found no trace of gas.
This had led them to believe that the harmful substance had dissipated by the time they had arrived.
It is still unclear just how long the boat had been moored at the island.
Norfolk Fire and Rescue and the East of England Ambulance Service and four boats from the coastguard all attended the scene after the alarm was raised.
On Thursday police had cordoned off a stretch of river around Wroxham Island, and it has now been re-opened.
The deaths caused shock and disbelief around the Wroxham community.
Norfolk county councillor for Wroxham Tom Garrod said he would be looking to what the police found out to see if there were any 'lessons to be learned' from the tragedy.
Anyone with information about the deaths should contact the police on 101 or, anonymously, Crimestoppers on 0800 555 111.
Visit the Gas Safe Register for more information on carbon monoxide safety at www.gassaferegister.co.uk
Gas - the silent killer
Although carbon monoxide poisoning has not been established as the cause of the Broads deaths, it would not be the first time the invisible, toxic gas has proved fatal in Norfolk.
In 2011, 30-year-old Hazel Woodhams died from gas poisoning poisoning when she and her partner Roland Wessling were camping at Clippesby Hall campsite, near Great Yarmouth.
The couple had used a barbecue in the evening and later brought it into the tent to protect it from rain.
Though the barbecue was out, it was still leaking gas which filled the tent. By morning, Miss Woodhams was dead and Mr Wessling nearly lost an arm due to poisoning.
In 2003, Keith Reynolds, 17, and Michael Frosdick, 19, died of carbon monoxide poisoning at their Great Yarmouth flat after a faulty gas fire had been fitted.
In February this year, Norfolk couple Ken and Linda Lincoln were found dead in a flat on Cyprus after a gas leak from a suspected faulty heater.
The couple lived on the island, and had strong links to the Wymondham area.
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