Japanese police have identified a 61-year-old man as a prime suspect behind a fire that engulfed a mental clinic in an eight-storey building where he was a patient, killing 24 people who were trapped inside.

The government also announced plans to inspect tens of thousands of similar buildings nationwide.

Authorities believe the massive death toll at the downtown Osaka building on Friday was largely because the fire made its only emergency stairway unusable.

Osaka police, who are investigating the case as arson and murder, identified the man as Morio Tanimoto.

He is being treated in serious condition after he was rescued from the fire, police said. He has not been formally arrested or charged.

After verifying security cameras and searching his home, police said they suspect Tanimoto was responsible for setting fire to the mental clinic, an official at the prefectural police investigation department told The Associated Press.

Kyodo News said Tanimoto was a retired metal worker and his former employer at the factory where he worked between 2002 and 2010 described him as diligent and skilled.

Some of his neighbours interviewed by local media described him as a grey-haired man who used to bicycle and hardly spoke.

Nishi Umeda Clinic for the Mind and Body was on the fourth floor of an eight-storey building in Osaka’s bustling business district of Kitashinchi and was known for its support for mental health at work.

Police searched Tanimoto’s house on Saturday and found the clinic’s patient card.

Authorities are investigating how the smoke filled the floor so quickly that the victims became trapped.

The fire that burned just 270 sq ft of the floor near the reception was mostly extinguished within 30 minutes.

On Sunday, internal affairs and communications minister Yasushi Kaneko, who is also in charge of fire and disaster management, said he had instructed a nationwide inspection of about 30,000 commercial buildings with three or more floors but only one stairway.

Mr Kaneko said many victims could not escape and died because the Osaka building’s only stairway was not accessible due to the fire and they lost their way out. He said the ministry will set up a panel of experts to discuss safety measures.