He is suspected on probable grounds of aiding and abetting murder and attempted murder, the Prosecution Authority writes in a press release.
The prosecutor will decide by noon tomorrow, Thursday, whether to request that the man be detained or released.
An 18-year-old man has previously been arrested on suspicion of murder and attempted murder.
Police and prosecutors are very tight-lipped about the now-arrested man's role in the attack. According to information provided to Aftonbladet, police suspected early on that another person may have been involved in the attack. This weekend, police, according to the newspaper's sources, carried out a raid in the Stockholm area and arrested a person suspected of involvement.
A 17-year-old girl was killed and three teenage boys were injured in the school shooting.
