Police suspect a 61-year-old male patient of starting a fire at a mental clinic in Osaka that killed 24 people, investigative sources said Saturday.

The police found the man's registration card for the clinic during a search of a place related to him, the sources said.

The man, who the sources say is in serious condition, is among 27 people taken to the hospital after the fire Friday raced through the clinic in a multiple tenant building in the western Japan city.

Photo taken Dec. 17, 2021, shows a building in Osaka where a fire broke out earlier in the day. (Kyodo) ==Kyodo
Firefighters provide first aid to those caught in a fire that broke out on the fourth floor of a building in Osaka on Dec. 17, 2021. (Kyodo) ==Kyodo

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24 killed after fire at Osaka mental clinic, arson suspected

Investigators enter the site of a fatal fire in downtown Osaka on Dec. 18, 2021. (Kyodo)

On Saturday, police and firefighters conducted a joint investigation into the site, located on the fourth floor of the building in Osaka's Kita Ward, to determine the cause of the fire.

The police also searched a house believed to be the man's and where another fire broke out about 30 minutes before the building fire.

"No one knows him around here," said one of residents living near the house in Nishiyodogawa Ward in the city. A part of the floor of the house was burned in the fire, the police said.

A woman in her 80s who lives nearby said the house had been vacant for many years, but recently a man was seen going in and out of the house. "I asked him, 'Have you moved?' and he said, 'Yes'," the woman said.

The police are investigating the incident as a case of suspected murder and arson. They were also still trying to confirm the identities of the 24 victims -- 14 men and 10 women.

The man is believed to have ignited a highly inflammable liquid he had stashed in a paper bag near the entrance of the clinic around 10:15 a.m. Friday, according to the sources.

Firefighters found most of the victims unresponsive when they reached the site.

The police said they have yet to contact the clinic's director Kotaro Nishizawa, leading investigators to believe he might be one of the victims.

Keita Suzuki, one of Nishizawa's acquaintances, said, "He is not the kind of person to earn the enmity of others. I hope he is safe."

With flames and smoke billowing out of shattered windows, the blaze was nearly extinguished by around 10:45 a.m. Friday, after burning 25 square meters, about one-third of the clinic's floor space.

Photo taken from a Kyodo News helicopter shows firefighters and police officers gathering in front of a building in Osaka where a fire broke out on the fourth floor on Dec. 17, 2021.  (Kyodo) ==Kyodo

The police quoted witnesses as saying Friday that the suspect had placed the paper bag near a heater in the clinic's reception area. He then kicked the bag, igniting the fire when the liquid flowed out.

The victims, aged around their 20s to 60s, may have died from carbon monoxide poisoning.

On Friday, the clinic was scheduled to hold a group session for patients on temporary sick leave to help them get back to work.

Some of the victims may have been there to attend the session.