An expert witness who becomes an active participant in a criminal investigation, rather than maintaining impartiality, risks having their evidence assessed as of little or no evidentiary value on any controversial issue. Where a circumstantial murder case depends substantially on compromised expert evidence, weak identification evidence, and speculative motive, the verdict cannot stand. The prosecution's failure to disclose communications between the expert and police that revealed the expert's investigative role and partiality contributed to the miscarriage of justice.
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