STOCKHOLM >> One brutal attack by a man who drove a stolen truck into shoppers in Stockholm has brought Sweden’s open-door immigration policies under increased scrutiny — and raised the question if Swedish society, considered democratic and egalitarian, has failed to integrate its newcomers.

The suspect in Friday’s attack, a 39-year-old native of Uzbekistan who has been arrested by police, had been on authorities’ radar previously but they dismissed him as a “marginal character.” It was unclear whether he was also a Swedish citizen or resident or even how long he’d been in the country.

The attack killed four people and wounded 15. In response, hundreds gathered today at the site of the crash in the Swedish capital, building a heartbreaking wall of flowers on the aluminum fence put up to keep them away from the site’s broken glass and twisted metal. Some hugged police officers nearby.

“We have been too liberal to take in people who perhaps we thought would have good minds. But we are too good-hearted,” said Stockholm resident Ulov Ekdahl, a 67-year-old commercial broker who went to the memorial.

Joachim Kemiri, who was born in Sweden to a Tunisian father and a Swedish mother, says migrants and refugees had been arriving in too large numbers.

“Too many of them have been coming in too fast,” the 29-year-old railway worker said. “It’s too much.”

Sweden has long been known for its open-door policy toward migrants and refugees. But after the Scandinavian country of 10 million took in a record 163,000 refugees in 2015 — the highest per-capita rate in Europe — Prime Minister Stefan Lofven conceded it could no longer cope with the influx.

At a press conference in late 2015, deputy prime minister of the small Greens Party — a junior government partner — Asa Romson, broke into tears as she announced measures to deter asylum-seekers in a reversal of Sweden’s welcoming policy toward people fleeing war and persecution. She described it as “a terrible decision,” admitting the proposals would make life even more precarious for refugees.

Today, Lofven laid flowers at the truck crash site, declaring Monday a national day of mourning, with a minute of silence at noon. He urged citizens to “get through this” and strolled through the streets of the capital to chat with them.

No one has claimed responsibility for Friday’s attack but Sweden’s police chief said Saturday that authorities were confident they had detained the man who carried it out.

Uzbekistan and other former Soviet Central Asian republics have long been a fertile recruiting ground for Islamic militant groups, notably the Islamic Movement of Uzbekistan, which formed in 1998. Originally allied with al-Qaeda, many of the group’s fighters have switched to Islamic State group affiliation.

Russian officials say the suicide bomber who attacked the St. Petersburg subway on April 3 was a native of Kyrgyzstan.

Sweden’s police chief Dan Eliason said officers found something in the stolen beer truck that “could be a bomb” or an incendiary device, but said they were still investigating.

Although it was not clear how long the suspect had been in Sweden, the Scandinavian country prides itself on welcoming newcomers. Still, its open-door immigration policy and comparatively heterogeneous culture has led to frictions, sometimes urban unrest, especially in areas where many long-time immigrants feel disempowered.

The populist, right-wing Sweden Democrats have tapped into a growing anti-immigrant sentiment. Mattias Karlsson, the party’s parliamentary group leader, said Saturday that he feels “anger and sorrow but not shock” over Friday’s attack.

“Unfortunately, there have been clear signs that it was just a question of time before the next attack would hit Sweden,” he said. “It will have far-reaching implications for society and politics.”

Steve Eklund, an office worker a few blocks away from where the accident occurred, said Sweden’s immigration policy had gone wrong.

“Sweden has made some mistakes, and something needs to be done to assimilate the immigrants better,” Eklund said. “But it takes two to tango — the immigrants living here need to reach out to ethnic Swedes too.”

Not everyone agreed. Visiting the crash site, Sweden’s Crown Princess Victoria laid roses on the ground today and wiped away a tear.

“We must show a huge force, we must go against this,” she told reporters. “Swedish society is built on huge confidence, a sense of community.”

Joachim Lindstrom said he wasn’t surprised by the attack.

“I don’t think that Sweden has really failed in its efforts to assimilate newcomers,” he told the Associated Press. “We have had long experience with them, but much depends on the people themselves.”

In February, U.S. President Donald Trump shocked Swedes when he suggested that Sweden could be the next European country to suffer the kind of extremist attacks that have hit France, Belgium and Germany.

Friday’s attack was the latest in which drivers have used vehicles as weapons.

In an attack last month claimed by the Islamic State group, a man drove a rented SUV into a crowd in London, killing four people and injuring many others before stabbing a policeman to death. He was killed by police.

The IS also claimed responsibility for a truck attack that killed 86 people in Nice, France, in July 2016 during a Bastille Day festival, as well as another truck attack that killed 12 people at a Christmas market last year in Berlin.

Friday’s truck attack on Stockholm’s pedestrian shopping street of Drottninggatan was also near the site of a December 2010 attack in which Taimour Abdulwahab, a Swedish citizen, detonated a suicide bomb, killing himself and injuring two others.

The prime minister made a point today of walking around Stockholm, including along Drottninggatan, chatting with people having coffee outside a cafe. He said the aim of terrorism is to undermine democracy.

“But such a goal will never be achieved in Sweden,” Lofven said.

Others feared the deadly attacks could continue.

“Things like this will always happen in an open society. Sweden is not a totalitarian society,” Eklund said. “Maniacs can’t be stopped.”