The 39-year-old Uzbek suspected of carrying out the Stockholm truck attack had been refused residency in Sweden and had shown interest in extremism, according to police.
Here is what we know about Friday's attack on a busy shopping street in the Swedish capital -- the latest of several to have used a vehicle as a weapon.
- What happened? -
Just before 1300 GMT on Friday, a stolen beer truck barrelled for several hundred metres down Drottninggatan, Stockholm's biggest pedestrian street, before slamming into the front of the Ahlens department store.
Witnesses described scenes of panic and horror as authorities quickly sealed off the area.
The attacker fled the scene, with police arresting the Uzbek man on Friday night.
On Saturday police said they had found a suspect device in the cab of the truck.
Swedish authorities said four people were killed -- a Briton, a Belgian and two Swedes -- and that children were among the 15 injured.
Nine people remain in hospital, four of them seriously injured, according to health authorities.
- Who was the attacker? -
Authorities have yet to name the man suspected of carrying out the attack, but police said Sunday that he was known to have "shown interest for extremist organisations" such as the Islamic State group.
The Uzbek national had also been refused residency in Sweden, having been warned in December 2016 that he had a month to leave the country.
But by February, he had "gone underground" and his case was handed over to police, police chief Jonas Hysing told reporters in Stockholm on Sunday.
A second suspect, whose identity was not disclosed, was arrested on Sunday. That person can be detained until Wednesday, at which point prosecutors would have to ask a court for permission to extend their detention.
- Previous attacks -
The Stockholm attack followed a string of similar assaults in Europe using vehicles.
The deadliest came in France on the Bastille Day national holiday last July when a man rammed a truck into a crowd in the Mediterranean resort of Nice, killing 86 people.
Last month, Khalid Masood, a 52-year-old convert to Islam known to British security services, drove a car at high speed into pedestrians on London's Westminster Bridge before launching a knife attack on a policeman guarding the parliament building.
Five people were killed in the attack, while Masood was shot dead by police.
In 2010, another section of Drottninggatan was the scene of Sweden's only other terror attack, when a suicide bomber blew himself up, slightly injuring several others.