Senior Public Prosecutor Hans-Jörgen Hanström asked Stockholm District Court for an extension of the preliminary investigation until July 6th, and was granted his request on Thursday.
Uzbek national Rakhmat Akilov is being held in custody after confessing to driving the truck in the attack on Drottninggatan which killed five people and injured several more.
"Substantial investigative matters remain, such as further interviews, analyses, surveys and technical investigations, before the investigation can be completed," Hanström said in a statement.
The prosecutor has previously said that the investigation could take up to a year to complete. Akilov, 39, admitted to committing "a terrorist crime" during a remand hearing at Stockholm District Court on April 11th.