People were killed when a truck crashed into department store Ahlens on Drottninggatan, in central Stockholm, Sweden April 7, 2017.

The suspect in Stockholm's deadly beer truck attack is a 39-year-old native of Uzbekistan who had been on authorities' radar previously, Swedish authorities said Saturday. The prime minister urged citizens to "get through this" and strolled through the streets of the capital to chat with residents.

Swedes flew flags at half-staff Saturday to commemorate the four people killed and 15 wounded when the hijacked truck plowed into a crowd of shoppers Friday afternoon in Stockholm. Prime Minister Stefan Lofven declared Monday a national day of mourning, with a minute of silence at noon.

Sweden's police chief said authorities were confident they had detained the man who carried out the attack.

"There is nothing that tells us that we have the wrong person," Dan Eliason told a news conference Saturday, but added he did not know whether others were involved in the attack. "We cannot exclude this."

Eliason also said police found something in the truck that "could be a bomb or an incendiary object, we are still investigating it."

Prosecutor Hans Ihrman said the suspect has not yet spoken to authorities and could not confirm whether he was a legal resident of Sweden. Anders Thornberg, head of the Swedish Security Service, said security services were working with other nations' security agencies to investigate the attack, but declined to elaborate.

Police declined to comment on media reports about overnight police raids around Stockholm or if they were hunting any more suspects in the case. They said the suspect had been on their radar before but not recently, and did not explain why authorities apparently had not considered him a serious threat.

Eliason told reporters the suspect was "a more marginal character."

Sweden's health service said 10 people were still hospitalized for wounds from the attack and four of them were seriously injured.