An Italian man has been arrested after he was accused of using rat poison to murder his grandparents and aunt.

Police said Mattia Del Zotto used the toxic chemical thalium, which is found in rat poison, to kill his paternal grandparents and aunt. He was arrested in the town of Monzo, near Milan, on Thursday (7 November).

Prosecutor Maria Luisa Zanetti told reporters that Del Zotto had sprinkled the chemical over the food and in the drinks of his relatives.

She said his grandparents, aged 94 and 88, and his 62-year-old aunt died after eating the substance in October. Five other family members are receiving treatment in hospital.

Police discovered his motives after reading an email on his computer in which he mentioned the poison's price, as well as the shipping and tax rates.

After his arrest, the 27-year-old reportedly told police that he used the poison on his family members as he wanted to "punish unclean people," the prosecutor said.

Zotto's mother told police that her son had become "manic" in recent months and spent all his time playing computer games and drinking alcohol. He had reportedly said that people were only allowed to consume "vital" things.

His mother said she believed he was under the influence of a religious cult.

Thalium is a soft metal that can be dissolved in water. It is an odourless and tasteless substance.