From the New York Times:
Authorities believe the conspirators were building bombs to detonate in vans in a massive attack on targets in Barcelona, including the Sagrada Familia, the city’s most famous landmark, according to a U.S. security official who has read Spanish intelligence reports.
The vast church, likely the most famous building under construction in the world, is the masterpiece of Art Nouveau architect Antoni Guadi (1852-1926).
Although it long languished without funds to complete it over the last 135 years, it now gets 3 million visitors per year, which covers its construction budget.
Much of ISIS Era jihadi terrorism has taken place at tourist attractions. If you want a list of likely future targets, just Google Top European Tourist Attractions:
Gaudi’s church is #3 after The Eiffel Tower and the Colosseum. I suspect, though, that St. Peter’s would be high on the terrorism target list.