According to the Prosecution Authority, the suspect is being held on probable cause, the higher level of suspicion.
"The investigation is ongoing, but due to the confidentiality of the preliminary investigation, I cannot provide any further information about the case," says senior prosecutor Sara Malmhester, who is leading the preliminary investigation, in a press release.
Lawyer Erik Eckersund, who has been appointed by Nacka District Court to represent the arrested man, declines to comment on the case.
The fatal shooting took place at around 3 p.m. on Wednesday last week. Shots were fired at a bus with around 20 people on board while it was at a bus stop near a school.
The police have conducted a large number of interviews and secured video footage, including from the bus.
The shooting in Tyresö was one of five shootings that occurred within a few days. Four young men died in these incidents: three were teenagers and one was 20 years old.
