A truck plowed into a crowd on a shopping street and crashed into a department store in central Stockholm on Friday, killing four people and wounding 15 in what the prime minister said appeared to be a terrorist attack.

Swedish police said they had arrested one person after earlier circulating a picture of a man wearing a grey hoodie. They did not rule out the possibility other attackers were involved.

“We have a person who is arrested who may have connections to the event in Stockholm earlier today,” police spokesperson Towe Hagg said.

There was no immediate claim of responsibility.

“I turned around and saw a big truck coming toward me. It swerved from side to side. It didn’t look out of control, it was trying to hit people,” Glen Foran, an Australian tourist in his 40s, told Reuters.

“It hit people, it was terrible. It hit a pram with a kid in it, demolished it,” he said.

“It took a long time for police to get here. I suppose from their view it was quick, but it felt like forever.”

Part of central Stockholm was cordoned off and the area was evacuated, including the main train station. All subway traffic was halted on police orders. Government offices were closed. (Map of attack location - tmsnrt.rs/2oguW2M)

“Sweden has been attacked. Everything points to the fact that this is a terrorist attack,” Prime Minister Stefan Lofven told reporters during a visit to western Sweden. He was immediately returning to the capital.

Many police and emergency services personnel were at the scene, said a Reuters witness who saw policemen put what appeared to be two bodies into body bags. Bloody tyre tracks on Drottninggatan (Queen Street) showed where the truck had passed.

Reuters An ambulance at the scene of an incident.

The truck had been stolen while making a beer delivery to a tapas bar further up Drottninggatan, Spendrups Brewery spokesman Marten Lyth said. A masked person jumped into the cab, started the truck and drove away.

“We were standing by the traffic lights at Drottninggatan and then we heard some screaming and saw a truck coming,” a witness who declined to be named told Reuters.

“Then it drove into a pillar at Ahlens City (department store) where the hood started burning. When it stopped we saw a man lying under the tyre. It was terrible to see,” said the man, who saw the incident from his car.

Police said four people had died and 15 were injured. National news agency TT said those hurt included the delivery driver, who had tried to stop the hijack.

Arash Pendari runs Vionlabs, a startup located in an office building just a few hundred feet from the crash. He was working Friday when he heard a loud noise outside his window and saw people “running away in a panic” on the street below.

“Police came quickly and started leading people away,” Pendari, 43, told The Huffington Post on Friday. “There was blood on the ground.”

“We’re definitely shocked,” Pendari added. “We’re not used to this type of commotion, especially in the central city and on a Friday on such a crowded day.”

Several attacks in which trucks or cars have driven into crowds have taken place in Europe in the past year. Al Qaeda in 2010 urged its followers to use trucks as a weapon.

Islamic State claimed responsibility for an attack in Nice, France, last July, when a truck killed 86 people celebrating Bastille Day, and one in Berlin in December, when a truck smashed through a Christmas market, killing 12 people.

Magnus Ranstorp, head of terrorism research at the Swedish Defence University, told Reuters the attacker’s approach was similar to those in Berlin and Nice: “Hijacking a truck, that has happened before.”

“And this is a pretty cunning modus operandi,” he said. “To drive to Ahlens (department store) and stop ... There is a way down to the subway just a few meters away from there, and then you ... can jump on any train you want and quickly disappear.”

Sweden’s King Carl Gustaf said in a statement: “Our thoughts are going out to those that were affected, and to their families.”

“An attack on any of our member states is an attack on us all,” said European Union chief executive Jean-Claude Juncker.

Stockholmers opened up their homes and offered lifts to people who were unable to get home or needed a place to stay.

The attack was the latest to hit the Nordic region after shootings in Copenhagen, Denmark, in 2015 that killed three people and the 2011 bombing and shooting by far right extremist Anders Behring Breivik that killed 77 people in Norway.

Sweden has not been hit by a large-scale attack, although in December 2010, a man blew himself up only a few hundred yards from the site of the latest incident in a failed suicide attack.

In February U.S. President Donald Trump falsely suggested there had been an immigration-related security incident in Sweden, to the bafflement of Swedes.

Swedish authorities raised the national security threat level to four on a scale of five in October 2010 but lowered the level to three, indicating a “raised threat”, in March 2016.

Police in Norway’s largest cities and at Oslo’s airport will carry weapons until further notice following the attack. Denmark has been on high alert since the February 2015 shootings.

Neutral Sweden has not fought a war in more than 200 years, but its military has taken part in U.N peacekeeping missions in a number of conflict zones in recent years, including Iraq, Mali and Afghanistan.

The Sapo security police said in its annual report it was impossible to say how big a risk there was that Sweden would be targeted like other European cities, but that, if so “it is most likely that it would be undertaken by a lone attacker”.

(Reporting by Stockholm newsroom; Writing by Gwladys Fouche; Editing by Catherine Evans)

TT News Agency / Reuters Police on the scene.