The shots were fired around 1pm on Monday in an apartment in Sollentuna, a suburb in northern Stockholm.
Police were quick to the scene arresting the 18-year-old gunman and taking a further three people in for interrogation.
The victim of the shooting was rushed in an emergency helicopter to the Karolinska University Hospital.
At first, Swedish media reported that she had died late on Monday night from her injuries, which included at least one gunshot to the head. But police on Tuesday morning denied that she had died, saying she is still alive but in a “critical condition”.
The gunman, who was arrested while still armed and wearing only his underwear, is believed to have had a relationship with the girl, according to Aftonbladet.
“The victim and the suspect are not unknown to one another,” said Christian Jofur of the Stockholm police to the paper.
Police have not revealed whether the three people taken in for interrogation are suspects or witnesses, and have only stated that they may be able to offer information.
The crime has now been relabelled as a suspected attempted murder and police are continuing their investigations.