Photo/Illutration The building in Osaka's Kita Ward where an arson attack on Dec. 17, 2021, led to the deaths of 25 people (Takuya Tanabe)

OSAKA--The man behind an arson attack at a mental health clinic here late last year that claimed 25 lives spent months staking out the venue and planning every aspect of the crime in minute detail, according to investigative sources.

The suspect, Morio Tanimoto, 61, died of carbon monoxide poisoning on Dec. 30, having never regained consciousness after setting the Dec. 17 blaze that gutted the fourth-floor clinic and killed the doctor, staff and patients.

Osaka prefectural police found that Tanimoto inputted memos on his smartphone about daily comings and goings at the clinic and also searched the internet for others who had gained notoriety for killing large numbers of people.

Investigators found the smartphone while combing through the crime scene. Tanimoto signed a contract for it on May 23, 2021.

He used a scheduling app to record his observations as he staked out the site, which he often did on Thursday evenings and Fridays.

For example, on Sept. 9, a Thursday, he wrote that at 9:13 p.m., Kotaro Nishizawa, the clinic head, left the first-floor entrance of the building.

On Oct. 22, a Friday, as was Dec. 17 when the crime was committed, Tanimoto recorded that 22 people had gathered at the clinic by 9:58 a.m. and that a receptionist opened the door to the stair landing three minutes later.

The clinic held sessions on Friday mornings for those seeking to return to the workplace after overcoming their mental health issues.

Such a session was scheduled for 10 a.m. on Dec. 17 shortly before Tanimoto set the clinic ablaze.

Tanimoto was a patient at the clinic, having first visited in March 2017 when he said he couldn’t sleep and felt lethargic. He visited the clinic on 112 occasions.

Osaka police who viewed security camera footage of Tanimoto during his final visit on Dec. 3 before carrying out the attack said it apparently showed him applying caulk to a fire extinguisher at the clinic to render it difficult to use.

On the night before the crime, security cameras recorded the sound of someone fiddling with the emergency exit from the clinic a minute after it was vacated.

Among other details disclosed by investigative sources was the fact that Tanimoto’s bank book showed he had no savings by January 2021. Until then, he had survived on rental income from a property he owned in Osaka.

Tanimoto’s smartphone also had no numbers stored in its phonebook.

The last entries recorded on his smartphone on Dec. 17 showed that he was planning his crime until the very end.

One entry said, “Must confirm lighter works,” while another said, “Have to bring a knife.”

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