Carbon monoxide poisoning is thought to have killed a mother and her 10-year-old daughter on a boat in Cumbria.
Kelly Webster, 36, and Lauren Thornton, from the Leyland area of Lancashire, were airlifted to Royal Lancaster Infirmary where they both died.
Matthew Etefson, 39, also from Leyland and who owned the vessel which was on Windermere, is seriously ill in hospital after Monday's incident.
Post-mortem examinations are due to be carried out later.
It is understood one theory involves the use of a generator on the boat, which may have been faulty.
Cumbria Police said officers were called to reports that three people on the boat were having serious breathing difficulties.
A spokeswoman said: "Police are investigating the circumstances of this tragic incident and trying to establish the full facts about what has happened. The coroner has been informed."
The incident, at a jetty near Bowness, was witnessed by Dragon's Den star Duncan Bannatyne, who remarked on the police and ambulance activity to his followers on Twitter.
He later posted a photograph of the scene, saying: "Tragic accident over there I am afraid."
Other eyewitnesses reported seeing firefighters entering the boat wearing breathing masks.
Joshua Kynaston, from Bolton, said: "I was in an ice-cream shop in Bowness when I heard ambulances.
"Shortly after they started racing up and down the road until they stopped right in front of me. The medics jumped out and ran towards the boat.
"The medics jumped aboard the boat and pulled the victims off.
"From where I was, I could partly see past the sheet where the medics were still working hard and dragging defibrillators and other portable medical machinery from the ambulances.
"Someone was taken off in a helicopter. Another person had countless amounts of CPR.
"Some of the firemen started to board the problem ship to search for the leak. They were in full suits with air tanks, not oxygen tanks."
Nigel Wilkinson, from Windermere Lake Cruises, said public cruisers were diverted while the emergency services worked at the scene.
He said police, fire, ambulance, lake wardens, community first responders and the Great North Air Ambulance and the North West Air Ambulance were all involved.
He said there was a feeling of "great sadness" in the area.