Italy's top anti-terror prosecutor says four suspects arrested in northern Italy were "very dangerous."

Speaking on a radio show on Friday Franco Roberti said however that investigators hadn't found "any trace that attacks plans were starting to be carried out" by the four, who were arrested a day earlier. He also says no arms or explosives were found.

Roberti says the probe leading to the arrests "confirms it's true what the United States is saying," referring to U.S. intelligence warning to Italy about the presence of terrorists ready to strike in Italy.

The prosecutor says "there's no doubt, the danger is there."

Italian investigators have said Italy, because it hosts the Vatican, can be an Islamic State group target.