Italian police have arrested three Kosovars in the city of Venice and say one was caught on a phone intercept proposing they bomb the Rialto bridge.
A minor was also detained in the early-morning sweep on Thursday.
Venice prosecutor Adelchi d'Ippolito said a search of an apartment showed the suspects were getting in physical shape and watching videos of Islamic extremists on how to carry out knife attacks.
A phone intercept caught one saying: "You'll go straight to paradise because of all the infidels in Venice. Put a bomb on the Rialto."
It was not clear if the reference was bluster or indicated an imminent threat.
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The 16th-century Rialto is one of Venice's iconic tourist draws, spanning the Grand Canal and featuring small shops on either side of the steps of the arched bridge.
Venice Mayor Luigi Brugnaro said those detained had regular residency permits to live in Italy.
He said as well as the detentions, police searched 12 sites in Venice's centre, Mestre and nearby Treviso.
