Prosecutors said Thursday that Mattia Del Zotto confessed after police searched his home and found vials of thallium, a colorless, odorless and tasteless heavy metal that is highly toxic. The search also turned up purchase receipts for thallium.
Monza prosecutor Luisa Zanetti quoted Del Zotto as saying he wanted to "punish impure" people. His mother reportedly told investigators she believed he was under the influence of a cult.
Italian media have been speculating for weeks about the mysterious poisonings after traces of thallium were found in tea the family drank at their home near Milan. The grandparents fell ill at the end of the summer and died in October; some relatives remain hospitalized with neurological problems.
Carabinieri Capt. Manusueto Consentino said investigators were still trying to understand the motive. He said Del Zotto wasn't under psychiatric care or known to be religious, but his family concurred he had grown more "introverted" lately.
Thallium was once used as a rodent killer, but the World Health Organization 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 behavior 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.
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