The house fire that left two women and three children dead was "severe", a spokesman for South Yorkshire Fire and Rescue said.
He said: "Our investigators are arriving here at 9am, so we we will know more later on this morning."
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Five people were killed and dozens more were injured after a car and knife attack in the capital on Wednesday afternoon.
Among those expected to attend the ceremony will be former US President Bill Clinton and ex-Northern Ireland first minister Peter Robinson.
Five people have been killed and 40 injured after a car and knife attack in London on Wednesday.