The 67-year-old suspect, who is known to the hospice, is ill in hospital.
He was initially arrested for arson with intent to endanger life on Saturday but was then further arrested on two counts of suspicion of murder on Sunday (July 12).
This was after two hospice residents, a woman aged 62 and an 81-year-old man, who were being treated for smoke inhalation at the Conquest Hospital following the fire, died.
Another woman resident aged 78 also died on Saturday.
However her death is not believed to be connected to the fire.
Fire, ambulance and police all declared the fire a major incident and 23 residents and nine staff were all evacuated.
Detective chief inspector Mark Preston, of the Surrey and Sussex Major Crime Team, said: “The arrested man is unable to be questioned currently due to his ill health.
“This is a complicated investigation and further enquiries are being carried out to establish the cause of death of the residents.
“This is an extremely difficult time for the residents and staff of St Michael’s Hospice and for the families of the residents who have sadly passed away.”
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