Emergency crews were called to the blaze at Woodlands View care home in Stevenage, Hertfordshire, at 16:30 GMT on Saturday February 3.
Hertfordshire County Council said there had been a “minor explosion” caused by an oxygen cylinder.
A resident believed to be 80s who was in the room where the fire started died in hospital.
Another resident is in a critical condition and five others are being assessed in hospital.
The home in Magpie Crescent is owned by HC-One. A spokesman said: “Our deepest sympathies are with their family and we are doing everything we can to support them.
Darryl Keen, chief fire officer and director of community protection at Hertfordshire County Council, said: “Hertfordshire Fire and Rescue Service was called to reports of a fire in one area of Woodlands View Care Home in Stevenage at 4.30pm yesterday (Saturday, February 3) afternoon.
“Eight fire engines were sent to the scene.
“There were 41 people in the premises at the time of the fire, many of these self-evacuated but firefighters completed five rescues and assisted another 10 from the property.
“All 41 residents, staff and visitors, were assessed by the ambulance service. The fire was out by 5pm.
“Seven people were taken to hospital and sadly one man in his 80s has died. One person remains in a critical condition in hospital.”
A Hertfordshire County Council spokeswoman said there appears to have been a “minor explosion caused by an oxygen cylinder”, but stressed a full investigation needs to be completed.
A spokesperson for HC-One, which owns the home in Magpie Crescent, said it is doing everything it can to support residents’ families, with additional staff from across the organisation arriving quickly to help the effort.
“At this stage we do not know why yesterday afternoon’s incident took place and in due course, there will be a full and proper investigation,” they added.
“Our immediate priority remains providing all support possible to everybody connected to the home and affected by yesterday’s events.”