Swedish police have confirmed they discovered a suspect device inside the lorry which was driven into a Stockholm department store on Friday.
The device was found in the driver’s seat, National Police Commissioner Dan Eliasson said, but it was not known whether it was a bomb.
Mr Eliasson also said the suspect in custody was from Uzbekistan, 39, and known to security services.
The hijacked lorry was driven into Ahlens department store in the capital.
Four people were killed – 10 remain in hospital, including a child. Two are in intensive care.
What do we know about the suspect?
The suspect, who was not named, was known to the security services.
However, he had only been seen as a “marginal character”, Mr Eliasson said.
The head of Swedish security police, Anders Thornberg, said the suspect was “a person who has previously figured in our intelligence flow”.