The police found explosives in the truck used in the attack in Stockholm on Friday, Swedish television said on Saturday citing multiple unnamed police sources.
Swedish public broadcaster SVT said the police had found a bag containing explosives.
The head of the Swedish security police told Swedish TV4 that he could not confirm whether explosives had been found.
Swedish police on Saturday also said they believe they had arrested the driver of the hijacked beer truck which killed four people and injured 15 others on a busy street in the Swedish capital.
“Yes, it is correct, it is likely him,” police spokesman Lars Bystrom said.
The stolen beer truck travelled for more than 500 yards along a main pedestrian street known as the Drottninggatan before it smashed into a crowd outside the upscale Ahlens department store about 3pm on Friday.
It came to rest in the entrance to the building. TV footage showed smoke coming out of the store after the crash.
Prime Minister Stefan Lofven said on Friday that the attack “indicates that it is an act of terror.”