The boy was initially arrested, but the arrest was later lifted because he is under 15 years old.
He is being cared for by social services, says Christian Magnusson, acting police chief in Helsingborg.
It is unclear how the boy feels about the criminal suspicion.
"It's nothing I can comment on," says Magnusson.
He also does not want to comment on whether the girl and the boy were in a relationship.
We initially assess it as an isolated incident and at this time there are no more suspects.
The incident occurred in Furutorpsparken in Helsingborg at around 3 p.m. on Monday.
Helsingborgs Dagblad has spoken to witnesses to the incident who say they heard screams from the park and saw an injured girl.
The girl was taken to Helsingborg Hospital with serious injuries. Her condition is now stable.
She is in good health, says Magnusson.
