The first murder took place in January. Six weeks later, a man who had witnessed that murder - and had been in touch with the police - was killed in broad daylight in front of the police; his wife was also killed.
Eight young men are on trial for the murders.
The ordeal has led to questions about whether witnesses should be able to testify anonymously, which is currently not allowed in Sweden.
Police officer Mikael Hiljegren, who has worked with protecting witnesses for more than two decades, told Swedish Radio that he believes witnesses should have the right to testify anonymously. He has observed witnesses being harassed in court, when people in the viewing gallery catch their eye and make violent gestures.
But the Swedish Bar Association is opposed to the idea of anonymous witnesses. The association's secretary general Anne Ramberg said she believes that would jeopardize the rule of law, because defense attorneys would not be able to assess the credibility of witnesses before cross examining them.