Uzbek national Rakhmat Akilov, 39, told police investigators that he had “achieved what he set out to do” and freely admitted his guilt, according to a local newspaper report.
He also said he was a part of the terrorist group Islamic State (ISIS) and he had received orders from the group’s high command in Syria.
No terror group has claimed responsibility for the attack.
Rakhmat Akilov is said to have carried out the Stockholm terror attack
Akilov was arrested on Friday evening in a northern suburb in the north of the Swedish capital after he hijacked a beer lorry and used it to plough through a crowd of pedestrians in the heart of Stockholm before crashing it into a shopping centre.
According to one report Akilov was so keen to tell officers he had links to ISIS he made the claim as he was being wrestled to the ground Märsta on Friday evening.
He reportedly said: “I bombarded Sweden because you bomb my country.”
The Stockholm Terror attack – in pictures
Fri, April 7, 2017
The vehicle drove into crowds on Drottninggatan (Queen Street) – one of the city's major pedestrian streets, just before 3pm local time, before crashing into Åhlens Mall
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Truck rams into crowds in central Stockholm
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I bombarded Sweden because you bomb my country
Rakhmat Akilov's reported words
A former co-worker of Akilov contacted by Reuters said Akilov had never expressed any religious views and also said the man he shared a flat with near Stockholm was said to be "devastated" to hear Akilov may be responsible for the attack.
Akilov's former co-worker, who had worked with him at a construction company in Sweden and did not wish to be named, said Akilov was a father who sent money home to a wife and several children living in Uzbekistan.
He said: "He never expressed any radical or religious views. He was like any normal guy.”
A man is arrested on the streets of Stockholm after the terror attack
Police have also confirmed the killer had an explosive device beside him in the cab of the lorry made up of gas bottles, chemicals, nails and bolts.
The perpetrator killed four people and injured another 15 in the attack, including Briton Chris Bevington, 41, who was working in Stockholm for the online music streaming site Spotify.
The others dead are a female Belgian tourist, 31, and two Swedes including an 11-year-old girl who was walking home from school. Police said all have been identified and their relatives informed.
Rakhmat Akilov is believed to be responsible for the Stockholm terror attack
Before the page was taken down Akilov’s Facebook profile showed he followed a group called "Friends of Libya and Syria", dedicated to exposing "terrorism of the imperialistic financial capitals" of the United States, Britain and Arab "dictatorships".
He also liked Playboy magazine and Russian tennis player Maria Sharapova, according to his social media page.
Officials had been trying to deport him from the Nordic country after his application for a work permit was denied in 2014.
The Briton Chris Bevington killed in the Stockholm terror attack
The aftermath of the Stockholm terror attack
His extremist sympathies had been known to the authorities but no action was taken because they did not believe he was “high-risk”.
The man had also managed to avoid authorities, having given them the wrong address.
Akilov has been charged with terror offences.