An arsonist is believed to have started a factory fire which saw 50 people evacuated from their homes in Oxford because of fears a gas cylinder would explode.

Richens Care's premises, in Windmill Road, Headington, were burned out on Saturday. The firm supplies mobility aids for the disabled, including wheelchairs and scooters.

Initially, it was thought the fire could have been caused by a lightning strike, but yesterday police said arson was suspected.

Detectives and fire investigators were combing the scene for clues yesterday. from their homes for 25 hours as firefighters cooled down the cylinder of acetylene. They said the 6ft cylinder would have exploded like a bomb if they had not managed to keep it cool.

Det Insp Simon Morton, of Oxford CID, said: "We're working closely with the fire service to ascertain the cause of this fire.

"The seat of the fire appears to be at the rear of the premises.

"We thought it was one of the fires started by lightning. Upon investigation, it became evident that it was arson and not a lightning strike.

"There are tell-tale signs when a building is struck by lightning and they weren't evident in this building. We were told by the fire experts that it was unlikely to spontaneously combust, due to fire precautions in the building, which would indicate this was done on purpose."

Oxfordshire Fire and Rescue Service's risk reduction manager, Peter Cleary, said: "If this was deliberate, it was completely inappropriate to endanger so many people's lives. If it was arson, then it's a police issue."

He added: "This is a very extensive fire and it's going to take a few days for us to excavate and investigate to find out the exact cause."

Mr Cleary said investigators would have to take the building apart piece by piece. The roof and first floor of the premises have collapsed.

Fifty people in Windmill Road and Kennett Road were evacuated to Wood Farm Community Centre and only allowed back to their homes at 3.20pm on Sunday.

Oxfordshire County Council's emergency planning officer John Kelly praised them for cooperating with the emergency services.

He said: "The unsung heroes of this incident were the residents, who complied with all instructions, looked after themselves and helped the emergency services.

"They were calm and unruffled and can be proud of the way they conducted themselves in a less than ideal situation."

A 55-year-old man was arrested on suspicion of arson in connection with the fire. He was released later without charge.