TOKYO—Japanese police on Sunday identified a 61-year-old man as a prime suspect behind a fire that engulfed a mental health clinic in an eight-story 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, which 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. Security camera footage broadcast by NHK television showed what it said was the suspect heading to the clinic Friday morning, carrying a backpack and riding a bicycle that had bags in a front basket …