It is believed a truck ploughed into pedestrians before ramming into a department store in the city centre.
An eyewitness told local media that someone jumped into the truck driver's cab and drove off with the vehicle while the driver was unloading.
Head of Swedish police's National Operations Department (NOA) Mats Lofving released a picture of a key suspect, taken from CCTV footage, close to the time and location of the attack.
The image shows a man wearing a jacket with a dark hood over a bright t-shirt and dark trousers.
Police say they have arrested a man in connection with the attack but will not confirm if he is the supected driver.
Swedish Prime Minister Stefan Lofven says the attack is likely to have been an act of terrorism.
He says Sweden would strengthen its border control in response.
