Earlier, prosecutor Hans Ihrman said a person had been formally identified as a suspect "of terrorist offenses by murder", and should face a pre-trial custody hearing before midday Tuesday or be released.
An Australian tourist has described witnessing a stolen truck ploughing through a crowd into a department store in central Stockholm, killing four people and wounding 15, in what appears to be a terror attack.
The Swedish Police were informed of the incident around 2.50pm (local time).
Another witness said: "I went to the main street when a big truck came out of nowhere".
"I couldn't see if anyone was driving but it was out of control". "We rushed down the street, and I glanced to the right and saw the truck".
"I ran as fast as I could".
Television footage showed a column of smoke rising from the scene and helicopters flying overhead. He "may have connections" to the attack, police said, but they provided no details.
While it's unclear who was responsible or what the motives were, it's a scene that's becoming all too familiar in Europe.
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. "We want to get in contact with this person", he told a news conference. Authorities evacuated the city's Central Station, which serves regional trains and the Swedish capital's subway system, which was a few hundred yards away.
The BBC video also showed that people running in fear from the incident in the Drottninggatan area of Stockholm.
Witness Jan Granroth told Swedish newspaper Aftonbladet, that "we stood inside a shoe store and heard something ... and then people started to scream". Last month, in an attack claimed by the Islamic State group, a man drove into a crowd on London's Westminster Bridge, killing three people and injuring many others before stabbing a policeman to death.
The usually busy pedestrian mall has been locked down by police and citizens have been urged to avoid the area.
A spokesperson for Spendrups brewery in Sweden confirmed to ABC News that the truck used in the alleged attack was stolen from them.
Several people were also injured in the incident on Drottninggatan (Queen Street), one of the city's major pedestrian streets.
The attack followed a string of assaults in Europe by people using vehicles as weapons. I also saw three others injured.
The vehicle bomb never went off and, after setting fire to the Audi, he was unable to detonate the other two explosives as planned.