The flames spread quickly through homes during the early hours Firefighters have confirmed that a blaze which swept through a village terrace was deliberately started.
Seven houses were well alight when South Wales Fire and Rescue Service arrived early on Saturday.
A woman remains critically ill after the fire in Railway Terrace in Blaengarw, in Bridgend county.
Police are continuing investigations into the exact cause of the fire and forensic officers have visited the scene.
A spokeswoman for the fire service said: “We believe it was deliberate but the investigation is now in the hands of the police.”
The fire at 0034 GMT on Saturday saw 20 people evacuated from eight homes in Blaengarw.
One house was severely damaged.
Fire service officer Tim Pendrick, the commander at the scene, praised his team for preventing the spread of the flames.
Scenes of crime officers began work with the fire investigation team on Saturday to establish a cause.
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