Four people were killed, including two Swedes, one Briton and one Belgian, when a truck mowed down pedestrians on a busy shopping street in central Stockholm on April 7th.
READ MORE: All articles related to the Stockholm attack
Rakhmat Akilov, a 39-year-old Uzbek national, confessed to driving the stolen truck and is being held in custody but investigators are still searching for possible accomplices.
The second unnamed suspect was arrested on Sunday, the prosecution authority said in a statement, adding: “No more information can be given at the moment.”
The prosecution has until Wednesday to decide whether the suspect should be formally remanded in custody.
Police said they had jointly carried out the arrest with the Swedish Security Service “after a large amount of preliminary investigations had been processed”.
No terror group has claimed responsibility for the rampage, which resembled vehicle attacks in Nice, Berlin and London.
Construction worker Akilov is a father of four who was refused permanent residency in Sweden in June 2016. He went underground last year after receiving a deportation order, police said.
He is known to have sympathies for jihadist groups, including Isis.
The second suspect was released late on Tuesday. Prosecutors say he is no longer a suspect.