Rakhmat Akilov reportedly ran from the scene still covered in blood and glass, and was arrested hours later in a northern suburb of Stockholm.
He has yet to be identified by police, who have only said that the man in custody is a 39-year-old Uzbek national.
Even so, a picture of the man allegedly behind the attack has started to emerge: someone who had failed in his bid to get residency, lost his job and was hiding from police who wanted to deport him.
Bumping into a former colleague earlier this year, he revealed he was spending his days "sleeping and smoking".
According to reports, he had left a wife and four children behind in Uzbekistan in order to earn money to send home.
He had applied for residency in 2014, but had been informed in December 2016 that "he had four weeks to leave the country", police official Jonas Hysing said. He did not leave and, in February, was officially put on a wanted list.
A few months earlier, it is claimed he had lost his job after falling asleep at work.
He had been working in construction, and was employed by Pierre Svensson for several weeks late last year, helping on an asbestos removal project.
Mr Akilov was, he said, "a reserved person".
"He didn't stick out. He did his job. You can't say he was very sociable, we just told him what to do and he did it. He didn't speak much Swedish," he told news agency AFP.
Mr Akilov was also described as not being particularly religious. One has suggested he "partied and drank", which goes against strict Islamic doctrine.
"He never talked about politics or religion," one friend told Swedish daily Aftonbladet. "He didn't pray five times a day from what I know."
A co-worker agreed, telling news agency Reuters: "He was like any normal guy."
Online, it seems, it was a different story.
His Facebook page - which has since been taken down - is linked to a number of extremists through friends and featured at least two propaganda videos linked to IS, one reportedly showing the aftermath of the Boston bombing.
He also liked a page called "Friends of Libya and Syria", which says it aims to expose "terrorism of the imperialistic financial capitals" of the US, British and Arab "dictatorships".
However, he was also a fan of pages dedicated to Playboy magazine and Russian tennis player Maria Sharapova.
Despite all this, Mr Akilov was not considered a threat by Swedish security services, who dismissed him as a "marginal character", apparently on the fringes of larger extremist movements.
The main suspect of ploughing a truck into a department store in central Stockholm, killing four people, had been denied residency in Sweden and had expressed sympathy for so-called Islamic State (IS), police and reports said.