Doug Stanglin and Daniel Silberstein

USA TODAY

STOCKHOLM — Police arrested one man in connection to the attack that killed at least four people Friday when a large beer truck slammed into an upscale department store in a busy Stockholm pedestrian mall in what Swedish Prime Minister Stefan Lofven called an apparent "terror attack."

The Stockholm city council said another 15 were wounded, nine of them seriously.

“Sweden has been attacked. Everything points to a terrorist attack,” Lofven said.

Returning to the capital from central Sweden, the prime minister said the “the country is in a state of shock.”

He laid a bouquet of red roses and lit a candle near Ahlens department store, where the partially burned truck was still embedded in the entrance.

“The aim of terrorism is to undermine democracy," he said. "But such a goal will never be achieved in Sweden.”

Late Friday, police said they had started a “preliminary investigation into suspected terrorist crimes.”

Police released a blurry CCTV photo of the suspect riding an escalator and wearing a black, hooded sweatshirt and green jacket. Late Friday, police said that one person with "some kind of connection" to the attack had been arrested, but did not elaborate.

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The incident occurred shortly before 3 p.m. when the beer truck, hijacked by a masked gunman, careened down Drottninggatan (Queen Street) at high speed, leaving a trail of broken glass, tire tracks and bodies. After hitting the Ahlens department store, the truck burst into flames.

Filip Lundberg, 36, who works as a salesperson at Buttericks, a costume store on Drottninggatan, said he saw the attack but didn’t immediately realize it could be terrorism.

“I was right in front of the truck when it came charging down. I managed to get out the way,” Lundberg said. “We only realized exactly what had happened when there were helicopters and the police started streaming onto Drottninggatan. We locked up the store.”

After police shut down the street in the wake of the attack, Lundberg walked to a friend’s house on a neighboring island of the archipelago that comprises the Swedish capital. He described himself as traumatized. “I’m on the verge of tears,” he said. “This is unreal.”

With at least one suspect on the run, alarmed officials brought central Stockholm to a virtual standstill, with government offices on lockdown and bus and subways system halted. People were ordered to stay out of city center while thousands of workers already there were forced to walk home at the end of the Friday workday until the transit system was partially restored.

“There are police reports saying it is unsafe, but people are still moving in all directions as transport is shut off,” said Rachel Weiner, a Stockholm resident.

It's most significant attack in the country since December 2010, when an Iraqi-born Swede, Taimour Abdulwahab al-Abdaly, detonated two devices, including one that killed him, in the central Sweden.

Witness Jan Granroth told the daily Aftonbladet that “we stood inside a shoe store and heard something ... and then people started to scream.” He said: “I looked out of the store and saw a big truck.”

Leander Nordling, 66, told the newspaper that he was standing in the perfume department at Åhlens when he heard a loud bang.

"It sounded like a bomb had exploded and began to smoke heavily in through the main entrance," he said. Nordling said he and other customers and staff took shelter in a storage room.

Swedish beermaker Spendrups said its truck was carjacked earlier Friday, Aftonbladet reported.

"During a delivery to (a) restaurant, and while the driver was unloading, someone jumped into the driver’s seat and drove off with it," Spendrups communications director Marten Lyth told TT Swedish news agency.

Stanglin reported from Mclean, Va. Contributing: Dominic Hinde; Associated Press