Two 19-year-olds were called to a custody hearing on Friday in connection with the case.
The incident occurred at a private party attended by around ten young people of both sexes.
During the course of the evening, a group of young men turned on the 17-year-old and began sexually assaulted him in an attack classified by the police as rape. The victim was drunk at the time of the attack.
Six young men were arrested and detained on suspicion of rape. Four suspects were subsequently released.
"The suspicions against the four who were released still remain," police spokesman Torbjörn Carlsson told news agency TT.
A number of people witnessed the incident and police have not ruled out the possibility that there may have been more people involved.
"We'll see where it ends up. We're about to start investigating who did what," said Carlsson.
The police spokesman added that he had never before encountered a case in which a male victim was raped by several men.
"It is very unusual and an incredible violation for the 17-year-old," said Carlsson.
Police and prosecutors were refusing on Friday to reveal any more details, citing the victim's age and circumstance surrounding the case.