Two weeks ago in Saint Mark’s square in the tourist-laden city of Venice, would-be jewel thieves tossed a couple of smoke bombs into the busy selfie-taking crowd and tried to grab a handful of diamonds from a jewelry shop. People panicked, understandably assuming it was a terrorist attack, and undercover cops seemed to stream out of the marble. Within minutes, the security forces had determined that it wasn’t a terror attack, and calm was restored even before the smoke cleared.

The exercise proved two things. The first, of course, is that you can only steal jewelry under the cover of smoke bombs in movies. The second was that Venice security forces clearly considered their city on a list of soft targets for terrorists.

On Thursday, raids in 12 locations in the city and its suburbs that netted four alleged terrorists from Kosovo, including one minor, might explain just why they were so ready. All of those arrested were living in the city center near major tourist sites like the La Fenice opera house.

Police had the group under surveillance for nearly a year, after one of the men came back from Syria and raised the alarm at a local Islamic prayer center (there are no mosques in Venice) with fiery rhetoric.

But when the police overheard them celebrate the attacks that killed four near the U.K. Parliament in London last week, and started talking specifically about blowing up the Rialto Bridge that spans the Grand Canal, they decided to act. Whether the alleged terrorists were inspired by the bridge incident in London or had been planning such a soft target attack all along is the likely focus of investigators’ attention.

Among the wiretaps chief investigating prosecutor Adelchi d’Ippolito shared with journalists in a televised news conference was a disturbing exchange. “With all the non-believers there are in Venice, if you put a bomb under the Rialto Bridge you’ll immediately earn a place in heaven,” a suspect said, before pledging his unconditional support to the Islamic State.

D’Ippolito said it was just one of the many alarming comments overheard among the group. He also said that raids at locations frequented by the four men, who all had legal residency documents for Italy, showed evidence that they had been studying bomb making, though it was not immediately clear that they had the supplies needed to construct an actual bomb. They had also apparently been engaged in weapons training, including knife practice and physical endurance training.

Kosovo has its own problems with terrorist threats. In November, police thwarted a number of planned terrorist attacks with the arrest of 19 men who allegedly planned to target the Israeli national soccer team when it played in Albania.

As the busy summer tourist season approaches, which officially kicks off in Italy with Easter weekend, authorities are increasingly concerned that this will be another summer season of terrorists making surprise attacks on soft targets like last summer’s attacks in Nice and this month’s attack in London.

While authorities across Europe vow that they are doing what they can to root out terrorist cells, it is becoming increasingly obvious just how difficult it is to spot radicalization. In Italy alone, more than 400 men in Italian prisons are believed to be radicalized, but there seems little to do to stop them unless and until they make a false move once they are released.

In December, Anis Amri, the author of the Berlin Christmas market massacre was shot dead when he returned to Italy apparently trying to find sympathetic connections in the country. Amri was radicalized in a Sicilian prison where he spent four years.

Venice mayor Luigi Brugnaro gratefully thanked Italian security forces for “breaking up a dangerous and active jihadist cell in the center of the city.” But the obvious concern remains, if there can be such a group hiding in plain sight in one of the tourist capitals of Europe, where else are they?