Updated at 6 p.m. ET

On Thursday afternoon in Barcelona, Spain, the tranquility of a tree-lined pedestrian pathway was shattered by a bloody vehicular attack that killed more than a dozen people and prompted a manhunt. The driver's whereabouts were still unknown as of early Friday local time, police say.

A white van drove into a crowd of people on the famous Las Ramblas boulevard, killing at least 13 people and injuring at least 100, according to the Catalan government.

The driver of the vehicle fled the scene. Police say they have detained two people who are connected to the attack — but that neither is the driver.

As of 8 p.m. local time (2 p.m. ET), officers were still evacuating people from the Las Ramblas area. A man was killed at a police checkpoint after running over two police officers, who survived, but police say there is no evidence that was connected to the van attack, the BBC reports.

Regional police are describing the deadly incident on Las Ramblas boulevard as a terrorist attack but emphasized they do not know the motive.

The Islamic State has claimed responsibility for the attack, the SITE Intelligence Group reports.

As NPR previously reported, an ISIS claim of responsibility can mean a number of things. Some claimed attacks were planned by ISIS leaders, while others were planned independently. This time, the ISIS statement said that the "perpetrators" were "from the ranks of the Islamic State" and were responding to calls for attacks.

The van slammed into scores of people as it drove down the famous pedestrian walkway that runs down the middle of Las Ramblas. "It wasn't slowing down at all. It was just going straight through the middle of the crowd, in the middle of the Ramblas," an eyewitness told the BBC.

Keith Fleming, who lives in Barcelona, saw the aftermath of the attack from his balcony. He told The Associated Press that he saw "women and children just running."

The AP spoke to other witnesses:

" 'I heard a lot of people screaming and then I saw the van going down the boulevard,' another witness, Miguel Angel Rizo, told The Associated Press. 'You could see all the bodies lying through Las Ramblas. It was brutal. A very tough image to see.'"Jordi Laparra, a 55-year-old physical education teacher and Barcelona resident, said it initially looked like a terrible traffic accident." 'At first I thought it was an accident, as the van crashed into 10 people or so and seemed to get stuck. But then he maneuvered left and accelerated full speed down the Ramblas and I realized it was a terrorist attack. He zigzagged from side to side into the kiosks, pinning as many people as he could, so they had no escape,' Laparra said."

Nafees Hamid, a terrorism researcher based in Barcelona, was at the Placa de Catalunya, a major square on Las Ramblas, at the time of the attack. "I was just roaming around the area a little bit, actually just taking in some of the summer buzz here in Barcelona," he told NPR. "I just saw people running ... hundreds of people running in one direction and I heard police sirens coming from every single direction."

Videos posted on social media show the panic in the area.

On Twitter, the Catalan police force described the incident as a "massive trampling ... by a person with a van." Authorities urged everyone in Barcelona to avoid the area and remain calm. Nearby transportation stops, shops and restaurants closed down.

Spanish Prime Minister Mariano Rajoy said on Twitter that he is in contact with authorities and that the priority is "tending to the wounded in Las Ramblas and facilitating the work of security forces."

Investigations are still ongoing. Late in the evening on Thursday, police tweeted that the van attack is related to an explosion that happened on Wednesday night in Alcanar, Spain, a town more than 100 miles down the coast from Barcelona. That explosion, which killed one person, had previously been assumed to be caused by a gas leak.

Barcelona is the capital of Spain's autonomous Catalonia region, which has a distinct language and culture. Las Ramblas (also known as La Rambla), which includes a tree-lined pedestrian path, terminates in the Placa de Catalunya — a large public square in the city center, famous in its own right and well-visited as a transit hub.

Authorities advisedanyone in the region to stay where they are and communicate via social media instead of telephone to avoid overwhelming the phone system.

Spanish Prime Minister Mariano Rajoy declared a three-day mourning period in remarks delivered around midnight local time.

"We have fought many battles against terrorism throughout history. We have always beaten them. In this moment, the Spanish will win again," he said. "Today, the fight against terrorism is the principle priority of free and open societies like ours. It is a global threat and the answer must be global."

International leaders have condemned the attack, with British Prime Minister Theresa May saying, "The UK stands with Spain against terror," and President Trump tweeting that the U.S. "will do whatever is necessary to help. Be tough & strong, we love you!"

Vehicular attacks have grown increasingly common as weapons of terror.

Massive car attacks struck Berlin and the French city of Nice in 2016. Hamid, the terrorism researcher based in Barcelona who witnessed people fleeing the attack, noted that "since 2014 ISIS has been calling for people to get behind a car and to carry out attacks."

The tactic has also been replicated by other groups, he said, like the man who attacked a mosque in Finsbury Park in London. This past weekend, a man considered a white supremacist drove his car into a crowd of anti-racist protesters, killing one, in an incident U.S. authorities described as domestic terrorism.

Last December, Aki Peritz reported for NPR that it's difficult to prevent such attacks: "The potential targets are too numerous to defend them all," he says, and "a vehicle attack takes only one determined attacker."

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