Officials in the autonomous Spanish region of Catalonia, which includes Barcelona, said they had arrested two people for the attack in the city's Las Ramblas district, but not the driver.
The attack appeared related to an explosion the night before in a province about 125 miles south of Barcelona. It wasn't clear whether authorities had identified any others still at large.
In the early hours of Friday, local time, Spanish police confirmed a separate suspected terror attack in Cambrils, a coastal town south of Barcelona. Four alleged attackers were killed and another was seriously injured, police said. Details of the suspected attack were not immediately clear, but authorities said the situation was "under control."
Here's what we know so far:
- Thirteen people are confirmed dead, and more than 100 injured
- The van's driver is not in custody
- ISIS has claimed responsibility, but offered no proof
- Police say they have killed four people in Cambrils, a coastal town south of Barcelona, in response to a separate suspected terror attack.
- The attack is connected to a Wednesday night explosion outside Barcelona
- The United States has urged Americans in Barcelona to avoid Las Ramblas
"My most serious condemnation to the terrorist attack in Barcelona," Catalonia's Interior Minister Joaquim Forn tweeted just before night fell on the city.
The ISIS-affiliated Amaq news agency claimed responsibility for the attack, said Ken Wolf, a senior analyst at Flashpoint. But the group has produced no evidence in support of the claim.
The attack unfolded during the height of the tourist season in Spain's second-largest city, along a boulevard teeming with locals, tourists, merchants and street performers. The pedestrian plaza where the violence took place is known as La Rambla, and it's in a district known as Las Ramblas. It was the deadliest terror incident in Spain since March 2004, when nearly 200 people were killed by bombs planted on four rush hour commuter trains in Madrid. And it was the sixth deadly attack in Europe involving a car or truck in the past 13 months; the others occurred in Nice, Berlin, London and Stockholm.
Spain's history of terrorism goes much further back. The country battled a decades-long campaign by the Basque separatist group ETA, which relinquished its arms earlier this year.
Photo Gallery: Barcelona Attack: Pedestrians Flee After Van Rams into Crowd
Since the 2004 bombings, authorities have arrested dozens of people suspected of terrorism, including members of a suspected ISIS cell in June. Because of its proximity to North Africa, Spain is seen as a transit point for radicalized Islamic fighters. In 2015, authorities broke up an alleged ISIS-linked group they said was planning an attack in Madrid. In March, authorities seized thousands of weapons they said were bound for terrorist groups.
Following Thursday's attack, Spanish authorities and the U.S. consulate in Barcelona asked people to avoid the Ramblas area, particularly near Plaza de Cataluna.
U.S. Secretary of State Rex Tillerson expressed condolences for the victims and said the United States was monitoring the situation and offering assistance to local authorities. He urged Americans in Barcelona to let loved ones know whether they were safe.
President Donald Trump tweeted condemnation of the attack and urged Spain to be "tough and strong."
Video and photos of the aftermath showed victims, bloodied and unmoving, sprawled on the mall amid debris and scrambling emergency workers. Police moved through nearby streets with guns drawn.
Rachel Mersky, a product designer from Oakland, California, told NBC News she was walking in the area and heard a commotion. "Suddenly everyone starts screaming and running and falling over each other and crying, so clearly I started running too," she said.
Pau Barrena / AFP - Getty Images
She said she followed police closer to the scene but it had been blocked off by then.
Steve Garrett, a British tourist, watched the scene unfold from the roof of a bakery.
He recalled seeing two waves of screaming people, including many tourists and children, rush into a market area. Then came the police.
"They looked like they were sweeping the market area. It was quite clear they were looking for somebody or something," Garrett said.
Susan McLean, a former police officer from Australia, saw the rush of panicked people as well.
“I was a police officer for 27 years and the fear on their faces … I just knew,” McLean said.
She said she joined about 100 other people who sought refuge in a women's clothing store. After about a half hour, she returned to her nearby hotel room.
“It’s the sad reality of life,” she said of the attack. “It’s not a question of if, it’s a matter of when you will be affected by a terrorist attack.”
Several American men's college basketball teams were in Barcelona for a series of exhibition matches. Some, including Clemson University and Oregon State, have reported that their teams are OK.
Jack Davey was in Barcelona with family to watch her brother, who plays for the University of Arizona. They got caught in the post-attack stampede, and met with other American families who'd been at the scene of the crash.
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“They were sitting right there and they saw everything. They said it crashed right into the market and killed everyone. They were inconsolable,” Davey said. “We hugged each other and we prayed together for a while.”
They made it back to their hotel.
“People keep asking me, ‘Are you OK?’" Davey said. "Let me be clear. We are safe, but we are not OK. There’s no way to reconcile this right now.”