A suspect device was discovered inside the lorry that was driven into a Stockholm department store in a terror attack on Friday, police have confirmed.
The “technical device” was found inside the driver’s seat of the large truck, Swedish national police commissioner Dan Eliasson said.
It was impossible at this stage to know what the device was, only that “it should not be there”, he added.
The suspected terrorist, who was arrested and remains in custody, is a 39-year-old man from Uzbekistan who was known to security services.
The commissioner said that, although he was on their records, he was only seen as a “marginal character.”
He remains officers’ prime suspect for the attack, which saw a hijacked lorry plough into the front of a shop in the pedestrian thoroughfare of Drottninggatan.
Four people were killed in the horrific assault and 10, including a child, remain in hospital. Two are in intensive care.
Swedish borders have been tightened at the request of the country’s Prime Minister Stefan Lofven – who call the incident a terrorist attack.
The prime minister said: "If it is a terrorist attack, and regardless of whether it was carried out by an organisation or a lone perpetrator, the aim of terrorism is to undermine democracy, to sow discord between people so that more people will begin to hate and distrust one another.
"But those kinds of acts will never succeed in Sweden. We know that our enemy is this kind of vile murderer - not one another. We will use all of Sweden's strength to track you down.
"Our message will always be clear: you cannot suppress us, you cannot control our lives, you will never win."
The latest outrage inflicted on the continent came just two weeks after similar tactics were used to attack London when Muslim convert Khalid Masood drove into crowds on Westminster Bridge.
The bloodshed also bore hallmarks to attacks seen in Nice and Berlin last year.
Swedish border controls were reinforced following the attack and investigators remained at the scene after the lorry was removed.