Swedish authorities arrest the man they believe drove a truck into pedestrians on one of Stockholm's busiest shopping streets, killing four people and injuring 15 others.

Flags are flying at half-mast across Stockholm as the city mourns after a truck attack killed four people and wounded 15 others.

Police said they have the suspected driver of the stolen truck in custody following his arrest on suspicion of committing a “terrorist crime through murder”.

According to media reports, a man arrested on suspicion of terrorism offenses after the Stockholm truck attack is from Uzbekistan and was known to intelligence services.

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, according to CNN.

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.

Swedish authorities’ focus is reportedly now to prevent further attacks and investigate if there were other accomplices.  Security has been stepped up and border controls are in place.

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.