This elderly man with Alzheimer's killed his granddaughter. He says he doesn't remember

Fukui Station and surrounding area in Fukui City, Fukui Prefecture on March 29, 2022.

Tokyo (CNN)It began as a family argument and ended in tragedy -- a teenage girl stabbed to death by her elderly grandfather in a case that shocked a nation.

In a court in western Japan last month, Susumu Tomizawa, 88, admitted to killing his granddaughter Tomomi, 16, nearly two years ago -- but, he said, he doesn't remember doing it.
Tomizawa has Alzheimer's, a progressive and irreversible neurological disorder that destroys neurons and shrinks regions of the brain. In court, his lawyers argued he should not be held criminally responsible because his illness causes dementia, a condition marked by multiple cognitive deficits such as memory loss.
    "He was insane at the time due to dementia and alcohol consumption ... and therefore pleaded not guilty," they said.
      But the court in Fukui city disagreed.
        On May 31, Tomizawa was to sentenced to four-and-a-half years in prison for murder.
        The case shocked many in Japan -- an aging nation where the number of elderly dementia patients is rising.
          The trial, live-streamed from court, was closely watched and drew sympathy from many who expressed pity for Tomizawa and the family's loss of Tomomi.

          Stabbed to death

          Tomizawa and Tomomi had been living at his home in Fukui city, the court heard.
          On the night of September 9, 2020, they got into an argument that resulted in the teenager's death.
          Tomizawa recalled drinking heavily that evening. Upset and intoxicated, he took a 17-centimeter- (nearly 7 inches) long kitchen knife and entered Tomomi's bedroom, where he repeatedly stabbed her in the neck, the court heard last month.
          The alarm was raised when Tomizawa called his eldest son, saying he'd found Tomomi's bloodied body, the court heard. Police arrived on the scene soon after and arrested the elderly man.
          Tomizawa's mental state was a major focus in his trial as doctors, lawyers and judges debated whether or not he had knowingly killed his granddaughter.
          Doctors who assessed his condition insisted he had a motive for committing murder. "His actions were purposeful and consistent with his intent to kill," forensic psychiatrist Hiroki Nakagawa told the court.
          Prosecutors said the elderly man was able to control his actions and "possessed the ability to judge right and wrong," despite his illness.
          In its ruling, the court acknowledged Tomizawa's Alzheimer's, but said he had understood the weight of his actions. "After careful examination and consultation with the defendant, we [made] a careful judgment," judge Yoshinobu Kawamura said.
          "The defendant was in a state of mental exhaustion at the time of the crime and he had great difficulty in judging right or wrong or in dissuading himself from committing the crime -- but he was not in a state where he was unable to do so."

          Disease of the mind

          Alzheimer's is the most common form of dementia affecting elderly people, according to experts.
          "It is a degenerative brain disease," said Jason Frizzell, a psychologist who specializes in criminal court cases. "In virtually all cases, there is a gradual decline in a person's abilities over time."
          The disease attacks the brain and memory loss becomes worse as it progresses. Symptoms like paranoia, agitation, confusion and even violent outbursts are likely to occur, said Frizzell, who is also a professor at Arizona State University.
          "Of course not every patient will [display] the same set of symptoms. Situational context may also play a role in aggression -- whether a patient feels fearful of places or people they do not recognize," he said.
          Jacob Rajesh, a senior forensic psychiatrist at the Promises Healthcare facility in Singapore, said in cases of rapidly progressing Alzheimer's "it will be difficult to provide an accurate account of what actually happened."
          "There is also the question of fitness to stand trial -- is a person fit enough to give evidence on the stand and plead guilty or not guilty?" he said.
          Crimes involving dementia patients are also extremely complex, experts said.
          "How much of their conduct can we reasonably explain through the disease itself as opposed to other motivations such as anger or retribution," Frizzell said. He also highlighted moral and ethical value judgments.
          "How do we effectively or reasonably prosecute someone who may be fully debilitated by their illness in just a few more years? Is being compassionate toward a convicted person with dementia at odds with the community's perception of justice?"

          'Prisons full of elderly inmates'

          Japan has one of the largest elderly populations in the world. More than 20% of its residents are over the age of 65, according to government records, and the number of Japanese centenarians is increasing.
          Dementia mostly impacts the elderly and there are believed to be more than 4.6 million people in Japan who are living with the condition. Experts say the number will rise significantly as the country continues to rapidly age.
          Violent crimes committed by Japanese dementia patients are rare but a case similar to Tomizawa's in 2014 saw a 72-year-old man with dementia strangle an 82-year-old woman to death in a hospice. He received a reduced jail term of three years due to his condition.
          "Prisons in Japan are full of elderly inmates suffering from dementia," said Koichi Hamai, a criminal justice expert and law professor at Ryukoku University in Kyoto. "The number of elderly prisoners is increasing and we have to take various measures to [address it]."
          Tomomi had lived with her grandfather in Fukui, one of Japan's least populated prefectures and where about one in three residents is age over 65, according to government figures.
          Details of their life were sparse but observers highlighted issues like aggression and domestic violence that Alzheimer's patients and their frustrated caregivers often faced.
            "Dementia patients are known to act out against the people looking after them, the ones closest to them," said Rajesh, the forensic psychiatrist.
            "Patients [like Tomizawa] need a lot of monitoring and management to be at home, and it wasn't immediately apparent he had any."