Police are treating a fire in a wheelie bin which spread to a bungalow within a Hartlepool elderly residents' complex as arson.
Officers on patrol spotted the fire at a semi-detached bungalow on Maxwell Road at about 04:30 BST.
Occupants of the affected building and the 11 neighbouring bungalows were led out. Nobody was injured.
Cleveland Police said they are keen to trace two people seen leaving Maxwell Road at the time.
Det Sgt Stuart Robinson, of Hartlepool CID, said: "A number of elderly residents were evacuated to a local parish hall this morning after a fire was discovered which soon spread from a wheelie bin to a bungalow, resulting in extensive damage to the exterior of the bungalow and smoke damage within.
"The evacuation caused great distress to the vulnerable occupants.
"If it hadn't been for a passing police sergeant who spotted the fire and acted quickly then who knows what could have happened. The incident could have led to casualties or even fatalities."
Cleveland Fire Brigade tackled the blaze and Transco was called out to deal with a subsequent gas fire.
The road was closed between Moffat Road and Owton Manor Lane for gas pipe repairs.
Housing Hartlepool, which owns the properties, said in a statement: "We are pleased no-one is hurt and we thank the emergency services for their swift response.
"We are helping the experts determine the cause of the fire and we will assist them in their investigation."