Stockholm (CNN) A man arrested on suspicion of terrorism offenses after the Stockholm truck attack is from the central Asian republic of Uzbekistan and was known to intelligence services, Swedish authorities said.

Suspicions about the arrested man have strengthened during the course of the investigation, Dan Eliasson, chief of the Swedish Police told a press briefing Saturday.

Police are investigating a "technical device" found in the vehicle used in the attack, he said.

"We confirm that we have found a device in the truck that doesn't belong there. We are now investigating its content," Eliasson said. "Whether this was a classic bomb or some sort of flammable device is now a matter for our analysis."

Prosecutor Hans Ihrman said the arrested man was a 39-year-old from Uzbekistan.

The attack Friday in the heart of Sweden's capital killed four people and injured about 15 more. Eight of the injured are still being treated in hospital, Eliasson said. Four of the injured, all adults, have serious injuries, Stockholm County Council said earlier. One child suffered minor injuries.

Earlier, Sweden's public broadcaster SVT reported that a bag of undetonated explosives had been found inside the truck, which was stolen minutes before the attack as it made a delivery at a restaurant. The attacker apparently suffered burns caused by the explosives, which did not detonate properly, SVT said.

Photos: Truck rams pedestrians in Swedish capital Swedish Prime Minister Stefan Lofven visits the scene of a suspected terror attack on Saturday, April 8. A day earlier, a truck crashed in front of a Stockholm department store, killing four people and injuring more than a dozen others. Hide Caption 1 of 16 Photos: Truck rams pedestrians in Swedish capital The scene of the attack in Stockholm. Hide Caption 2 of 16 Photos: Truck rams pedestrians in Swedish capital A woman cries after placing flowers at a makeshift memorial outside the department store in Stockholm. Hide Caption 3 of 16 Photos: Truck rams pedestrians in Swedish capital A police officer stands guard at the site of the attack. Hide Caption 4 of 16 Photos: Truck rams pedestrians in Swedish capital A woman tosses a flower onto the memorial in central Stockholm. Hide Caption 5 of 16 Photos: Truck rams pedestrians in Swedish capital Authorities work at the scene of the attack on Friday, April 7. Hide Caption 6 of 16 Photos: Truck rams pedestrians in Swedish capital A medical responder moves through the scene of the attack. Hide Caption 7 of 16 Photos: Truck rams pedestrians in Swedish capital Police respond to the scene of the attack. Hide Caption 8 of 16 Photos: Truck rams pedestrians in Swedish capital A couple embrace after the incident. Hide Caption 9 of 16 Photos: Truck rams pedestrians in Swedish capital People check on an injured person. Hide Caption 10 of 16 Photos: Truck rams pedestrians in Swedish capital Police officers secure the area outside the Stockholm Central train station. Parliament and the Stockholm subway were placed on lockdown. Hide Caption 11 of 16 Photos: Truck rams pedestrians in Swedish capital An injured person is put in an ambulance. Hide Caption 12 of 16 Photos: Truck rams pedestrians in Swedish capital A police officer stands guard near the scene of the attack. Hide Caption 13 of 16 Photos: Truck rams pedestrians in Swedish capital Onlookers at the scene were stunned and distraught. Hide Caption 14 of 16 Photos: Truck rams pedestrians in Swedish capital Local media reported that the truck was hijacked as it made a delivery nearby. Eyewitnesses said it barreled down Drottninggatan (Queen Street) before it crashed into the front of a department store. Hide Caption 15 of 16 Photos: Truck rams pedestrians in Swedish capital Security services launched an investigation into who was involved in carrying out the attack. Hide Caption 16 of 16

A manhunt was launched immediately after the attack. The suspect was detained in the early hours of Saturday (Friday evening ET) in Marsta, a town about 30 minutes' drive north of Stockholm near the city's main international airport.

The man has featured in previous intelligence reports from last year, Anders Thornberg, chief of the Security Service told the briefing. However, the security services were not able to confirm that intelligence, so he was not on their radar, he said.

Their focus is now to prevent further attacks and investigate if there were other accomplices, Thornberg said. Security has been stepped up and border controls are in place.

An image of the suspect released by police.

Eliasson said investigators were examining the suspect's social media accounts and contacts, but declined to be drawn on whether he had connections to ISIS or other terror groups.

"We do not know whether there are further persons involved in this act or not," he said. "But we are not excluding that. We are still working on a very comprehensive approach to see whether there is any possibility that further individuals are involved."

Eliasson said he saw clear similarities to the attack in London last month, in which a man plowed through pedestrians on Westminster Bridge before fatally stabbing a policeman outside the UK Parliament.

The prosecutor has until noon Tuesday to ask the courts to detain the suspect, said Karin Rosander, spokeswoman for Sweden's prosecutor's office.

'We are not afraid'

King Carl XVI Gustaf on Saturday expressed his condolences to the friends and families of the victims.

Speaking outside the royal palace in Stockholm, Gustaf said the nation was "shaken up" but urged people to seek peace.

King Carl XVI Gustaf of Sweden makes a statement on April 8, 2017 in Stockholm, the day after the truck attack.

"We have experienced other violence acts before and we have survived then, he said. "We will also survive now... Sweden is -- has been for a long time -- and shall continue to be a safe and peaceful country."

Drottninggatan (Queen Street) remained cordoned off Saturday morning, but the truck had been removed overnight from the building where it had been wedged. Heavily armed officers guarded the area and several police vans were present.

Scores of people came to lay flowers Saturday morning at a spot on Klarabergsgaten, near the crash site. The mood was subdued, with people seeming to be in quiet reflection.

Crown Princess of Sweden visiting scene of #Stockholm attack A post shared by Max Foster (@maxfostercnn) on Apr 8, 2017 at 1:34am PDT

Sweden's Crown Princess Victoria and her husband Prince Daniel visited the scene mid-morning. Princess Victoria, who was dressed in black and visibly moved to tears, was barely able to speak as she surveyed the area. "I feel an enormous sorrow and emptiness," she told reporters.

Prime Minister Stefan Lofven, who visited a short time later, said: "The goal is that we are supposed to be afraid, but Sweden has shown itself from its best side. That is a strength that no one can take away from us."

People lay flowers Saturday at the scene of a truck attack in central Stockholm.

Police consoled one crying man nearby as he walked up with a bouquet. Several fathers brought their children down to the barrier and explained gently what happened.

Habib Jlassi, a 29-year-old night bus driver who was there with his three-year-old daughter, told CNN it was important to pay tribute to those who lost their lives.

"I've come here to show that we are not afraid," said Jlassi, who lives in Stockholm and whose eyes appeared red from crying. "We are stronger than them."

Around the corner, in a square outside Stockholm Concert Hall, a flower, fruit and vegetable market was being set up. Some people bought flowers there before walking over to leave them by the cordon.

Debris still littered the ground around the crash scene and workers in neon suits carried items to a waiting skip.

Some shops opened Saturday morning on streets around Drottninggatan as the city center began to return to normal.