The day after an attack killed 13 people in Las Ramblas in Barcelona, locals try to make sense of the horror. (Raul Gallego Abellan/The Washington Post)

It was another mellow summer afternoon on a world-renowned boulevard, a site of teeming crowds and bustling cafes. Then there was a flash of white.

A rented Fiat van swung onto La Rambla, a broad, tree-lined pedestrian thoroughfare, and the driver pressed down hard on the gas. Almost immediately, he began to maim and kill.

“The van was just plowing down people,” said Carlos Tena, 34, a native of the city who was leaving work with two colleagues.

He watched as the van streaked by, “zigzagging right and left,” and then he saw what the driver was leaving in his path.

“I saw a little boy, in really, really bad shape, just lying there with his mother,” Tena said. “He was not moving. His mother asked me with her eyes if I could help.

Authorities in Spain have arrested four suspects after two terror attacks killed 14 people and injured dozens Aug. 17-18. (Sarah Parnass,Patrick Martin,Monica Akhtar/The Washington Post)

“My heart split in two,” he said. “I still don’t understand what I saw.” He said he did not know the boy’s fate.

The attack on Las Ramblas, as the district featuring Barcelona’s scenic, belle epoque promenade is known, was the worst terrorist attack in Spain since March 2004, when 192 people were killed and nearly 2,000 were injured in a coordinated series of bombings on the Madrid rail system. Thursday’s attack, which was claimed by the Islamic State, killed 13 people and injured more than 100 others from 34 countries. A separate vehicle attack south of Barcelona early Friday left another person dead and six injured.

The carnage in Barcelona on Thursday provided yet another example of a chilling new reality of urban life in Europe: ordinary vehicles suddenly transformed into weapons of mass murder.

For many who witnessed Thursday’s carnage, it was the image of the van — even more than the image of the driver — that remains most haunting.

What we know about the two vehicle attacks in Spain

Jose Moya, 51, was tending to a customer in his flower shop on the avenue, where he has worked for the past three decades.

On Friday morning, he was still shaking with emotion as he recalled the moment he first sensed, and then saw, the van.

“It passed so close to me,” he said. “It was a hand’s width from me.” Moya paused to hold his fist a few inches from his belly. Time seemed to speed up rather than slow down, he said. “It was moving so fast — really, really fast. . . . It took the people and dragged them.

“The people were flying!” he said.

“Look at me, I am still in shock,” a trembling Moya added.

Raquel Perez, 27, works at the Carrefour grocery store in Las Ramblas. She was on her break Thursday afternoon, eating in a little square at the end of a side street.

“First, I saw a lot of people running from Las Ramblas, and then suddenly stop.” she said. “Then I saw a second wave. Police followed and yelled, ‘Hide wherever you can!’”

So Perez, too, began to run.

“I was just running. I didn’t know what had happened. But then I started to see videos on social media, and there were all these rumors that there were guys with guns running around all over the place.”

It turned out that some of those initial rumors were not true. Early news reports mentioned a possible hostage situation, with a group of men holding others in a store.

But the driver of the van appears to have vanished. Spanish authorities have detained four others, and police were investigating the possibility that the driver was among five assailants killed early Friday in a second vehicle attack in Cambrils.

Perez and others found an open door, a bar closer to the sea. She and her friends jumped over the counter, hiding behind the bottles and glasses for hours.

“Today I was more afraid than yesterday,” Perez said on Friday. “I don’t know if something might happen again. And I fear that it will.”

Thousands of visitors shared the same experience.

The flash of white. The screams and confusion. The running and the hiding, often for hours, with no idea what was happening.

Many assumed that armed men were stalking more victims on the streets, as terrorist attackers did in Paris in November 2015.

“I was at home with doors and windows open, because of the heat. I heard a lot of shouting in the street. I thought it might have been celebration,” said Xochtl Martinez, 31, who has lived in the neighborhood all her life.

It wasn’t a party. “It was a stampede,” she said. “What I remember most are the faces. They were faces of pure panic. They were running in absolute terror.”

Spanish plainclothes and uniformed police shouted at the crowds to get indoors and stay in place.

Paulina Sanchez Avila, 18, was visiting from Mexico. She and a friend dashed into a Burger King at the top of the boulevard. Inside, panicking patrons tried to erect a barricade to prevent assailants from entering.

“People were throwing tables and chairs. Look at my shoulder,” she said, revealing a large, dark bruise. She and her friend stayed in the restaurant for five hours, then made their way to the Mexican Consulate, which kept them there until 3 a.m., when they were taken to a hostel. They went to the city’s memorial service later, red roses in hand.

Marga Soler, 46, was one of many merchants and small-business owners who sheltered tourists and pedestrians in their establishments. She said as many as 30 people pressed together in a small room.

Rajesh Kumar Sadhani, 48, was another. From India originally, he has lived in Barcelona for 18 years, working for the past six in a jewelry store in Las Ramblas.

When the attack began, he was helping clients who were looking at a watch on display in a case next to a window along the boulevard, he said. He heard a loud noise, but by the time he looked up, all he could see was a mass of people running by. Then, he said, there were the bodies left behind.

He and his colleagues sheltered 15 people for roughly five hours, until police released them around 10 p.m. He insisted that he was unafraid to be back at work on Friday. “I feel very well today,” he said.

By Friday afternoon, that sentiment was seemingly shared by many others. By midafternoon, cars were allowed to access the street, and Las Ramblas was again teeming with tourists and locals. Except for makeshift memorials of candles and flowers — as well as armed police and camera crews and armed police — it was difficult to tell that there had been a terrorist attack on the boulevard less than 24 hours earlier.

Writing in the early 20th century, the Spanish poet Federico García Lorca wrote that Las Ramblas was “the only street in the world which I wish would never end.” Many of the locals here insisted that it wouldn't.

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