He is suspected on probable cause – the higher standard of suspicion in Swedish law – of complicity in murder and complicity in attempted murder.
The prosecutor must decide whether to formally request the man be remanded custody or release him by midday tomorrow, Thursday.
An 18-year-old man, named in the Swedish media as Liam Nebel, has already been remanded in custody on suspicion of murder and attempted murder.
KEY FACTS: What we know about the deadly school attack in Sweden
Nebel allegedly entered the school using a key tag, and proceeded to attack people, seemingly at random, with a sword. He was arrested on site by police and placed in pre-trial detention on Sunday.
One 17-year-old girl died in the attack, with witnesses saying she had been stabbed repeatedly. Two boys, aged 12 and 17 were seriously injured with one requiring emergency surgery. A third teenager, also under 18, was left with more minor physical injuries.
According to the Expressen newspaper, Nebel had shared videos on TikTok and other social media extolling mass murderers and school attacks, including the shooting in Örebro in 2025, the attack at Malmö Latin in 2022 and Pettersson's attack in 2015. The account also celebrated the terror attack mounted by Anders Behring Breivik in Norway in 2011.
The account also reportedly shared content celebrating the anti-immigration Örebropartiet, the group Alternativ för Sverige, and other far-right content.