Swedish police say they believe a 39-year-old suspect arrested on Friday was behind the truck attack which left four dead and 15 injured.
Authorities declined to comment on reports that the suspect was a father-of-four from Uzbekistan.
The unnamed suspect reportedly confessed to the attack after being detained in Marsta, which is around 25 miles north of the Swedish capital.
Some reports suggested he had previously posted jihadist propaganda on his Facebook page and had images of people injured in the explosion at the Boston marathon in April 2013.
Police found explosives in the truck used in the attack in Stockholm, Swedish television said on Saturday citing multiple unnamed police sources.
Reports claimed the improvised explosive device (IED) was packed into a suitcase inside the hijacked beer truck.
If the reports prove accurate, it would clearly indicate that the terrorist was not a 'lone wolf' but part of a wider cell. and that the attack required planning. It is not clear why the IED, if it was in the truck, failed to detonate. The death toll would have been on a catastrophic level had it done so.
A Swedish police chief said on Saturday authorities are confident they have detained the man who carried out a deadly truck attack in Stockholm on Friday.
Dan Eliason, head of Sweden's National Police, told a news conference Saturday that "there is nothing that tells us that we have the wrong person."
He said police also found an object in the truck that "could be a bomb or an incendiary object, we are still investigating it."
Swedish prosecutor Hans Ihrman confirmed that the suspect is a 39-year-old Uzbekistan-born man but declined to name him.
The head of Sweden's domestic intelligence agency said the man had been on authorities' radar some time ago.
Anders Thornberg, head of the Swedish Security Service, said: "The suspect didn't appear in our recent files but he earlier has been in our files."
He said the security services are working with other nations' security agencies on the matter, but declined to elaborate.
Eight victims, including children, remain in hospital, according to Swedish media. Three of the victims died at the scene, a report said, while one died after arriving at the hospital.
Sweden’s prime minister, Stefan Löfven, said everything indicated the incident was terrorism.
It happened less than two weeks after the Westminster Bridge attack and stirred up memories of the attacks in Nice and Berlin where Islamist sympathisers used lorries as weapons – a tactic first suggested in a 2010 directive from al-Qaeda commanders to their supporters.
The attack also came less than two months after Donald Trump provoked a row with Sweden after suggesting that immigration had led to rising crime in the country.
Television footage showed hundreds of shoppers and office workers fleeing the scene after the lorry careered down the pedestrian precinct, killing a dog and crushing flowerpots and litter bins as it went.
“We stood inside a shoe store and heard something and then people started to scream. I looked out of the store and saw a big truck,” Jan Granroth told Aftonbladet.
Another witness said: “When I came out, I saw a lorry standing there, with smoke coming from it, and there were loads of bits of cars and broken flowerpots along the street.”
Annevi Petersson, a photographer, ran out from a store when she heard screaming coming from outside. “I saw a woman had a partly severed foot. People screaming in panic, others ran. I saw people laying bloody on the street and got out of there.”
Stockholm was put on lockdown, with the metro and mainline trains closed, as police fanned out across the city in pursuit of the suspect. Stockholm city council announced that it was opening public buildings for those stranded by the train and bus closures.