The man was arrested on Sunday as a result of in-depth analysis of evidence collected on the case, including new intelligence work by Sweden's security service Säpo, police said in a press release.
Prosecutor Hans Ihrman now has until Wednesday 26 April to convince a court to hold the suspect longer.
Police are unwilling to give further details, saying it is important to keep the investigation secret. The Prosecution Authority is even refusing to tell news agency TT which level of suspicion they have against the suspect. Such information is normally disclosed when police arrest a suspect.
Magnus Ranstorp, a terrorism researcher at Sweden's National Defence College told TT that it was "noteworthy" that neither the police not the lawyer were providing such basic information.
"It may be that they don't want to say anything because there may be further people who are suspected," he said.
On Monday Stockholm County Council announced that its hospitals were ending the state of heightened readiness they had entered after the attack on 7 April.
One person is still receiving medical care in the emergency ward, but not in intensive care.
The attack killed four people, including one child.
One man is already being held in custody, charged with being the driver of the truck. He has admitted to the crime.
In an earlier version of this article, we wrote that the suspect had been charged.
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