"About 100 investigators are involved in the investigation, and police do not rule out that it will take a year," Mats Loving told reporters adding that over 600 interrogations took place.
National Police Commissioner Dan Eliasson expressed confidence that the perpetrator of the attack has been detained.
"We are confident that we have exactly the man who was behind the wheel," Eliasson told reporters.
Police arrested two people, including a 39-year-old Uzbek national living in Sweden, who allegedly pleaded guilty to carrying out the attack. According to media reports, the man posted propaganda related to the Islamic State terrorist group, outlawed in Sweden, Russia and in other countries, on his Facebook page.
"Police are conducting the search of several other people whom we wanted to interrogate," the commissioner added.
On Friday, a stolen truck rammed into a crowd of people at a major pedestrian street in Stockholm, leaving at least four people dead and several others injured. The country's Prime Minister Stefan Lofven said that the incident was likely a terror attack.
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