A 27-year-old Italian has been arrested on suspicion of killing three members of his own family with thallium, because he claimed they were "impure".
Mattia Del Zotto, 27, said he killed them and used the poison on another five relatives "to punish impure people", Maria Luisa Zanetti, the prosecutor from the northern town of Monza just north of Milan, told reporters.
Del Zotto bought the thallium from a company in Padua near Venice, and sprinkled the toxic chemical in the drinks and dishes of his family members, the Daily Mail reports.
His paternal grandparents, Giovanni Battista Del Zotto, 94, and Maria Gioia Pittana, 91, as well as his aunt Patrizia Del Zotto, 63, died in October after ingesting the poison.
He then tried to poison other relatives. Five others, including his maternal grandparents, have been hospitalised in the past weeks.
Prosecutors said Del Zotto confessed after police searched his home and found vials of thallium, a colourless, odourless and tasteless heavy metal that is highly toxic.
Police also found an email on his computer quoting the poison's price as well as tax and shipping rates, investigators said.
His mother told investigators del Zotto had become "manic".
He spent his days in front of his computer and had stopped eating sweets and drinking alcohol, saying people were only allowed to consume "vital things.
She also reportedly told investigators he was under the influence of a cult.
"Lately he has been angry with the whole world. But I don't think he is capable of harming anyone," she was quoted as saying in Italian daily Corriere della Sera.
Carabinieri captain Manusueto Consentino said investigators were still trying to understand the motive.
He said Del Zotto was not under psychiatric care or known to be religious, but his family concurred he had grown more "introverted" lately.
Italian media have been speculating for weeks about the mysterious poisoning after traces of thallium were found in the tea the family drank.
Thallium, a soft metal, has long been used as a murder weapon as it dissolves in water and is odourless and tasteless.
The chemical was once used as a rodent killer, but the World Health Organisation in 1973 recommended it be discontinued because of its toxicity for humans.
Initial symptoms of thallium poisoning include gastrointestinal problems, delirium and coma.
Hair loss, psychotic behaviour and organ damage can also occur.
Poisoning can be treated with an antidote of Prussian blue, which prevents the thallium from being absorbed.
In the 1990s, there were several reports that Iraqi security agents were using thallium to poison enemies of Saddam Hussein.
The Agatha Christie novel The Pale Horse used thallium as its murder weapon of choice.