Police say potential carbon monoxide poisoning is their “primary line of inquiry”.
A woman has been arrested on suspicion of manslaughter following the deaths of three people at a care home.
Dorset Police said earlier the deaths at the Gainsborough Care Home in Swanage, were being treated as “unexplained” but potential carbon monoxide poisoning was the “primary line of inquiry”.
Emergency services were called to Gainsborough Care Home in Ulwell Road in Swanage, Dorset, at 05:20 BST.
Seven people were taken to hospital and residents were evacuated from the building to a nearby church hall.
Police said a 60-year-old local woman had been detained on Wednesday evening.
Det Ch Insp Neil Third said: “As part of our investigation, we have made one arrest to enable us to establish whether there have been actions or omissions which are grossly negligent and gather as much information as we can about what has happened.
“It is important to note that the arrest process also provides certain legal protection to individuals and I would ask for the media and public to avoid speculating any further now that legal proceedings are active.”
The force said earlier that the families of those who died have been informed and the coroner has been notified.
About 40 residents from the home, run by Agincare, were seen being transferred, some in wheelchairs, to All Saints Church a short distance away.
Dorset Council said it was working with the home and other agencies to support the residents of the care home.
“Many of the residents are vulnerable people with significant care needs, and so we are working with them to understand how to meet their needs in the short term and identify their next steps,” it said on Wednesday.
In an earlier statement, Agincare said it has allocated “extra staffing resources” to provide support after an incident at the site, and that residents moved to a village hall and church are “safe and well”.
They said: “We appreciate that this is a stressful and worrying time for those involved, and have allocated extra staffing resources to provide support locally. We are contacting and updating residents’ families.
“We will continue to work with the lead local authority services to return residents into their home at the earliest opportunity, and will provide further updates in due course.”
South West Ambulance NHS Trust said a hazardous area response team environment crew was sent to the scene along with 10 double-crewed ambulances.
Fire crews from across Dorset were also deployed. They were assisted by a crew from Winchester in Hampshire.
Gas distribution company SGN said it was called to Gainsborough Care Home earlier on Wednesday.
It said engineers were sent to complete safety checks in the building.
“Our thoughts are with everyone who has been affected by this tragic incident,” a statement said.
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