(CNN) Spaniards have expressed shock over two deadly terror attacks that hit the region of Catalonia just hours apart, having enjoyed years of calm since a decades-long separatist insurgency burned out.
But the attacks -- including one in Barcelona, in which a van rammed into a crowd and killed 13 people -- come against the backdrop of an increased extremist threat in Spain.
The Catalonia region, and Barcelona in particular, is the country's jihadist hub and has been for some time, experts say, and authorities have made dozens of terror-related arrests and foiled attack plots in recent years.
After a string of attacks in Europe and Tunisia, Spanish officials raised the country's terror threat level in 2014 and then again in 2015. It has sat on the second-highest level, indicating a "high risk" of an attack, ever since.
According to the Jane's Terrorism and Insurgency Centre (JTIC) database, Spanish police have arrested at least 20 suspects connected to ISIS across the country this year so far, 11 of them in Catalonia.
In 2015, 13 people in an ISIS-linked cell were arrested for preparing to bomb commercial areas and police headquarters in Barcelona, according to JTIC.
Fernando Reinares, director of the Global Terrorism Program at the Elcano Royal Institute, said that Barcelona is "the major radicalization pocket in the country."
"Since 2013, over a quarter of all individuals arrested in Spain for activities related to jihadist terrorism were residents of the province of Barcelona," Reinares told CNN, referring to the greater Barcelona area.
He added that half of Spain's Salafist population live in the city. Salafism is an ultra-fundamentalist branch of Sunni Islam that is particularly prevalent in Saudi Arabia.
Otso Iho, a senior analyst with Jane's, wrote in a briefing that unlike recent vehicle ramming attacks in London and some other European cities, the Spanish attacks indicate the involvement of wider network and more sophisticated coordination.
The Barcelona attack on Thursday was one of three related incidents that officials believe are connected.
On Wednesday, an explosion destroyed a house in Alcanar in southern Catalonia.
And in the early hours of Friday morning, another car ramming attack was launched in the coastal town of Cambrils. Police shot dead five suspects, whom they say were wearing fake suicide belts. Four suspects have also been arrested.
"The involvement of at least eight suspected perpetrators in three different events linked to the attacks suggests the presence of a wider cell and network of militants," Iho wrote, making his remarks before police announced a fourth arrest Friday.
"While no major terrorist attack has taken place in Spain since the 2004 Madrid train bombing, the country has an active network of Islamist militants and police have regularly disrupted plots and made terrorism-related arrests; illustrating a heightened threat in the country."
David Lea, a senior analyst for Europe with the Control Risks consulting firm, said that Barcelona had been for some time, like many multimillion-person cities, "a credible target" for an attack.
"This is in part because of its focus on tourism, its international fame and its recognizable buildings. The van stopped at the Miro mosaic, a photogenic target, which fits the criteria of terror attacks pretty well," Lea said.
He added that the threat in the city had been mounting over recent years as the number of arrests "have stacked up."
"There have been quite a few terror-related arrests in Barcelona -- mostly for extremist recruiting, funding and logistics in sending people to Syria. And a number of mosques there have been closed down as well."
But Lea warned that terror threats were consistent in most big Western European cities, saying that the next serious attack "could pop up easily in Birmingham, Berlin or Barcelona."