The witness had been called to testify at the trial of Jesse Torniainen, who was jailed for two years for an aggravated assault.
A key witness at a trial of National Socialist Jesse Torniainen failed to appear at a hearing in a court of appeals on Friday. The police were also not able to locate the witness at the address provided.
Last year Torniainen was sentenced to two years in jail for an aggravated assault at a demonstration in Helsinki. The victim later died in hospital although the court in that case did not agree with prosecutors that his death was down to the injuries sustained in the assault. Both the prosecutor and the defendant appealed.
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The witness, who was called to testify by both the prosecution and defense on Friday, had accompanied the victim on the day of the assault.
Torniainen is a member of the White National Socialist Nordic Resistance Movement, which a district court banned in November, calling the organisation a "violent group that exhorted followers and members to violent acts".