From 1999 to 2006, reports of threats in connection with court cases increased by 70 percent.
Since then, the rising trend has continued.
Last year 5,835 police reports were filed, up almost nine percent from 2010's figure. Roughly half of those reports involve youths between 15 and 25 years old, according to a study conducted by the Swedish National Council for Crime Prevention (Brottsförebyggande rådet - Brå).
"It's a threat against the rule of law," chief prosecutor Jörgen Lindberg of the Swedish Prosecution Authority's development centre in Malmö told news agency TT, commenting on the growing difficulties in convincing people to reveal what they've seen and heard.
According to Brå researcher Johanna Skinnari, attempts to influence witnesses and victims range from violence and threats to more subtle measures, such as wearing a motorcycle gang's vest in the courtroom, or staring aggressively at the witness.
Apart from the clear connection to criminal youths, threats and violence were common in trials of organized crime and relationship violence.
Skinnari warns that general feeling of discomfort in court cases can lead to self-censorship, stopping potential witnesses from participating.
Nearly all threats against witnesses turn out to be empty, and it is exceptionally rare for witnesses to get hurt.
"It's important that we shatter this myth that it's dangerous to testify. Some are just scared because they don't feel at home in the trial process, and are unfamiliar with the language and the environment, said Skinnari to TT.