Detective Sergeant David Speight says whoever caused the fires may have flash burns on their body.
Michael and Angela Broderick, who are both deaf, had a narrow escape when an arsonist destroyed their home in Burbage in Leicestershire.
Police are appealing for the public to contact them if they have any information.
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The missing 15-year-old boy was last seen on Monday 12 June close to a train station.
The 26-year-old had been spending the evening out with friends.
Hundreds of people have attended a Britain First rally in Birmingham this afternoon.