Swedish police have confirmed they discovered a suspect device inside the lorry which was driven into a Stockholm department store on Friday (07/04/2017).
The "technical device" was found in the driver's seat of the vehicle, National Police Commissioner Dan Eliasson said.
Mr Eliasson also said the suspect in custody was from Uzbekistan, 39, and known to security services.
Friday's attack saw a hijacked lorry driven into Ahlens department store on Drottninggatan (Queen Street).
Four people were killed - 10 remain in hospital, including a child. Two are in intensive care.