A 16-year-old boy has been detained on suspicion of murder since the end of July, and two other men have previously been detained on suspicion of aiding and abetting murder.
Now a fourth person has been detained on suspicion of aiding and abetting murder and a fifth has been arrested.
It was on March 28 that a 55-year-old man was shot in the head at a gym in Vasastan. He was taken by ambulance to hospital where he later died.