A suspect package has been found in the lorry driven into a Stockholm department store, Swedish police have said.
National Police Commissioner Dan Eliasson said a "technical device" was discovered in the drivers seat of the truck. The precise details of what the device is are yet to be ascertained but Eliasson said whatever it was "it should not be there".
Four people were killed and 15 wounded in what the Swedish Prime Minister said had appeared to have been a terrorist attack.
Earlier today, authorities said a man held in custody is understood to be the driver.
A truck, which had been hijacked, drove into a crowded shopping street and crashed into a department store in the centre of the Swedish capital.
Police released a security camera image of a man they wanted to question shortly after the attack. He was subsequently arrested in Marsta, a town 25 miles north of Stockholm.
"We have investigated several people this evening, who we found interesting in one way or another, and recently we arrested someone we are very interested in," said police investigator Jan Evensson.
And police spokesman Lars Bystrom subsequently told news agency AFP: "We suspect that the man who was arrested is the perpetrator."
Police have reportedly also made a second arrest.
"Our message will always be clear: you will not defeat us, you will not govern our lives, you will never, ever win," said Prime Minister Stefan Lofven, who had earlier described the assault as a terrorist attack.
However there has been no immediate claim of responsibility.
Police said security at Swedish borders had been heightened. They did not rule out the possibility other attackers were involved, according to reports by Reuters.