The Spanish region of Catalonia has been rattled by two terror attacks and an explosion over three days, in what authorities are treating as a trio of linked incidents.

On Friday, police said the perpetrators of the attacks in Barcelona and Cambrils originally planned to use explosive devices, but their plans were foiled after bomb-making materials accidentally blew up a house -- believed to be the suspects' base -- in Alcanar.

Four people have been arrested -- one in Alcanar and three in Ripoll. Three were Moroccan citizens and another was Spanish; they ranged in age from 21 to 34. None were on the radar for terrorism.

It is unclear how many people were involved in the attacks and how many suspects are still on the run.

Here's what we know so far:

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