Police control the area near to the scene following a suspected terror attack in central Stockholm, Sweden. Picture: AP

Stockholm – Swedish police are increasingly confident they have caught the man responsible for a truck attack in the capital Stockholm on Friday, but would not rule out the possibility that other people had been involved.

"Nothing points to that we have the wrong person, on the contrary, suspicions have strengthened as the investigation has progressed," the head of the national police, Dan Eliasson, told reporters.

"We still can not rule out that more people are involved." 

Police also said they were examining a device found in the driver's seat of the truck used in the attack in Stockholm which killed four and injured more than a dozen.

"We have found something in the truck, in the driver's seat, a technical device that should not be there. I cannot say at this stage that this is a bomb or some sort of flammable material," Swedish police chief Dan Eliasson told a news conference.

"Whether it is a classic bomb or a fire device or something else is subject to technical analysis," he said.