A 27-year-old Italian man has been arrested on suspicion of poisoning his entire family, leading to the deaths of his grandparents and an aunt and leaving five others in hospital.
Prosecutors said Mattia Del Zotto had confessed after police searched his home and found purchase receipts for thallium, a colourless, odourless and tasteless compound that is highly toxic.
Monza prosecutor Luisa Zanetti quoted Del Zotto as saying he wanted to "punish impure" people.
His mother reportedly told investigators he had joined a cult.
Italian media have been speculating for weeks about the mysterious poisoning after traces of thallium were found in the tea the family drank.
The grandparents fell ill at the end of the summer and died in October; some relatives remain in hospital with neurological problems.