PEOPLE FLEE IN Barcelona, Spain, after a white van jumped the sidewalk in the historic Las Ramblas district and crashed into a summer crowd of residents and tourists, killing 13 and injuring 100, police said. (AP Photo/Giannis Papanikos)

BARCELONA, Spain (AP) — Spanish police intensified a manhunt Friday for suspects behind two deadly vehicle attacks on civilians, shooting and killing five people wearing fake bomb belts who attacked a seaside resort and arresting four others believed linked to the carnage wrought on a Barcelona promenade.

Spanish authorities said the back-to-back vehicle attacks Thursday afternoon and early Friday morning — as well as a deadly explosion earlier this week in a house elsewhere in Catalonia — were related and the work of a large terrorist group.

The Islamic State group quickly claimed responsibility for Europe’s latest bout of extremist violence, which left 13 dead and 100 wounded after a van roared down Barcelona’s historic Las Ramblas promenade on Thursday. Hours later, a blue Audi plowed into people in the popular seaside town of Cambrils, killing one person and injuring five others.

Spanish Prime Minister Mariano Rajoy declared today that the fight against terrorism was a global battle and Europe’s main problem.

Police said they arrested two more people today, after an initial two were arrested Thursday — three Moroccans and one Spaniard, none with terror records. At least three of them were nabbed in the northern town of Ripoll. Another arrest was made in Alcanar, south of Barcelona, where a gas explosion in a house Wednesday that killed one person was also being investigated.

“There could be more people in Ripoll connected to the group,” regional Interior Ministry chief Joaquim Forn told TV3 television, adding that police were centering the investigation on identifying the five dead attackers in Cambrils as well as the driver of the Barcelona van.

Amid heavy security, Barcelona tried to move forward Friday, with its iconic Las Ramblas promenade quietly reopening to the public and King Felipe VI and Rajoy joining thousands of residents and visitors in observing a minute of silence in the city’s main square.

“I am not afraid! I am not afraid!” the crowd chanted in Catalan.

But the dual attacks unnerved a country that hasn’t seen an Islamic extremist attack since 2004, when al-Qaida-inspired bombers killed 192 people in coordinated assaults on Madrid’s commuter trains. Unlike France, Britain, Sweden and Germany, Spain has largely been spared, thanks in part to a crackdown that has netted some 200 suspected jihadis in recent years.

Authorities were still reeling from the Barcelona van attack when police in the popular seaside town of Cambrils, 80 miles to the south, fatally shot five people near the town’s boardwalk who had plowed into tourists and locals with their car. Forn said the five were wearing fake bomb belts.

One woman died today from her injuries, Catalan police said. Five others were injured.