A 42-year-old woman was taken into custody by prosecutors Monday on suspicion of murdering her 7-year-old son in 2019 by suffocating him.

The move came as police also look into the deaths of her three other children at an earlier date, all of whom also passed away at a young age.

Ayano Ueda, who describes herself as a nursing assistant, was arrested by police Sunday for allegedly suffocating her son Yudai by blocking his nose and mouth sometime between 8:45 a.m. and 3 p.m. at their home in Yamato, Kanagawa Prefecture, near Tokyo, on Aug. 6, 2019.

Investigative sources quoted her as saying the boy had chronic asthma when he was transported to a hospital.

They said it has been found that her eldest son and daughter died around 20 years ago, and another son passed away in 2017.

Police eventually suspected murder in Yudai's death given that his body had traces showing external force had been applied and after consultation with multiple doctors.

The boy, a first-grader in elementary school, is believed to have died of hypoxic encephalopathy, or brain damage caused by lack of oxygen, according to the sources.

Ueda has denied murdering her children, and no reports of her abusing them were received by police.

But Yudai was temporarily placed in the custody of a child consultation center twice -- the first time before the 2017 death of Ueda's other son, and the second time afterward. On the second occasion, he was returned to Ueda's care as a result of a decision by a family court.

She was suspected in the past of suffering from Munchausen syndrome by proxy, a mental health problem in which a caregiver makes up or causes an illness or injury in a child, elderly person or disabled person under his or her care to draw attention, according to local authorities.

The police are investigating whether the potential syndrome, which is also considered a form of abuse against a child or others, is related to the deaths of her four children.

Ueda, whose mental state at the time of her children's deaths is also being investigated, told her neighbors after Yudai died that her son was not breathing when she woke up in the morning and that the hospital had told her the cause of his death was unknown.

Ueda herself made the 119 emergency call for ambulance transport. His death was confirmed at the hospital, which later reported the case to the police.

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