One of the wounded, an 83-year-old Romanian woman who was begging on the city's pedestrian Drottninggatan shopping street when the attack took place, said she was "surprised" that passersby helped her.
Prosecutors and police officials did not identify the suspect, but Anders Thornberg, head of the Swedish Security Service, said at a news conference that the man had been on authorities' radar some time ago.
Swedish police have only identified the suspect as a 39-year-old man from Uzbekistan. "In February 2017, the case was handed over to the police to carry out the order, since the person had gone underground", he said.
The attack occurred at the Ahlens department store just before 3 p.m. Witnesses were quoted as saying the truck drove into the front window.
Four people were killed and 15 were injured when the lorry crashed into the front of a department store.
He says police also found an object in the truck that "could be a bomb or an incendiary object, we are still investigating it".
The Uzbek, detained on Friday night on suspicion of terrorist offences, appeared to have acted alone but police could not rule out that more people were involved, national police chief Dan Eliasson told reporters on Saturday.
"And he's been wanted by the police though he's not been in custody of the migration office", said National Strategic Commander Jonas Hysing. No identities other than nationalities have yet been made public. It said two were in intensive care, four were seriously injured but the one child was not seriously hurt.
More as we get it.
On Sunday, thousands of people gathered in central Stockholm for a "Lovefest" vigil against terrorism, and laid flowers outside the Ahlens shop in tribute.
Police say the man was known to have expressed sympathy for groups including so-called Islamic State. It is also a stark reminder of how hard it is for authorities to stop these types of attacks that are becoming increasingly common across the continet. The Briton killed in the attack has been named as Chris Bevington, 41, who had lived in Stockholm for 10 years.
A failed suicide bombing in December 2010 killed an attacker only a few hundred yards from the site of Friday's incident.