A second suspect has been formally arrested in connection with the attack that killed four people and injured 15 others, the Stockholm district court said on Sunday.
Judicial officials said a second person had been arrested in relation to the investigation into the attack.
The suspected driver, who was arrested hours after the attack, has been identified only as a 39-year-old man from Uzbekistan who had shown interest in extremist groups and who was facing deportation after his residency permit application was denied, police said.
"We know that he showed interest for extremist organizations like the 'Islamic State, '" said Swedish police chief Jonas Hysing. He was given four weeks to leave the country in December but could not be located because he had given police a wrong address.
A woman handed out roses to two police officers guarding the square: "thank you", she said and smiled.
Police Spokesman Lars Bystrom has since told news agencies, "We suspect that the man who was arrested is the perpetrator".
The beer truck, stolen earlier on Friday, mowed down pedestrians along Drottninggatan, a busy pedestrian shopping street.
Prior to Friday's attack, Sweden had escaped much of the violence which has afflicted other European capitals.
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On Sunday, people gathered in central Stockholm for a "Lovefest" vigil against terrorism, and laid flowers outside the Ahlens shop in tribute.
Police said they were unable to confirm local television reports of explosives being found inside the truck.
More as we get it.
However, police said they were ever more convinced that the Uzbek man was the driver of the commandeered truck and may have acted alone.
"I think it was just a matter of time, but still one doesn't think it will happen", Cecilia Hansson, a 25 year-old nurse, said.
A child is among ten people who remain in hospital, two of whom are in intensive care.
"We grieve with the families that have lost their loved ones, near ones, but we're also determined to continue to be an open society, a democratic society, and that is something I am totally confident that the Swedish people also feel", Stefan Lofven said.
A Downing Street spokesman said Theresa May had called the Swedish Prime Minister Stefan Löfven "to express her condolences and those of the British people for the awful attack that took place in Stockholm".