Osaka police have identified 61-year-old Morio Tanimoto as the prime suspect in Friday's fire at the clinic on the fourth floor of an eight-story building in Osaka's main business district of Kitashinchi.
The blaze also left two people severely injured, including Tanimoto.
The incident sent shockwaves across the country and recalled the deadly arson in Kyoto two years ago.
While searching the suspect's house, police found a months-old newspaper containing an article on the 2019 attack on the Kyoto Animation studio, indicating Tanimoto might have been inspired by it, an Osaka police investigator told The Associated Press on Tuesday on condition of anonymity because of department rules.
In the Kyoto Animation case, an attacker stormed into the building, spread petrol near the entrance and set the building on fire, causing an explosion and killing 36 people and injuring more than 30 others. The incident shocked Japan and drew an outpouring of grief from anime fans worldwide.
Tanimoto purchased 10 litres of petrol in November in apparent preparation, police said.
Tanimoto, who is severely injured and unconscious, is being treated in a hospital and has not been arrested or charged. One of the severely injured victims from the attack died in a hospital, bringing the fire's death toll to 25, police said on Tuesday.
Officials believe the death toll at the downtown Osaka building was high because the fire was started near the floor's only stairway, leaving many people trapped inside while smoke and heat quickly filled the space, the Fire and Disaster Management Agency said.
A security camera at the clinic showed a man walking into the reception area with a paper bag, which he put on the floor and kicked. As liquid poured out of the bag, he ducked down and moved his hand, and then a blaze shot up to the ceiling, with flames and smoke erupting.
In the footage, the man believed to be Tanimoto is seen moving farther into the clinic, where there was no emergency exit, the police official said, adding that it was not known whether he had a suicidal intent.
The skilled metal worker has had troubled relationships with his family and served a prison term for stabbing his own son in 2011 with a knife in a forced suicide attempt, Japanese media reported.
His brother told the media that Tanimoto had distanced himself from the family and they haven't been in touch for about 30 years.
Autopsy results showed most victims died from inhaling carbon monoxide and had few burns or other external injuries, police said Tuesday. The clinic's psychiatrist, Kotaro Nishizawa, was among the victims.
Witnesses and investigators have suggested that the victims struggled to find their way out of the clinic as they gasped for air. Most were found to have collapsed while headed to the other end of the clinic, only to find there was no alternative exit.
The government has launched a nationwide inspection of buildings with three or more stories and only one stairway as part of an effort to step up safety measures.
