Wis. Stat. § 972.09: Hostile witness in criminal cases.

Where testimony of a witness at any preliminary examination, hearing or trial in a criminal action is inconsistent with a statement previously made by the witness, the witness may be regarded as a hostile witness and examined as an adverse witness, and the party producing the witness may impeach the witness by evidence of such prior contradictory statement. When called by the defendant, a law enforcement officer who was involved in the seizure of evidence shall be regarded as a hostile witness and may be examined as an adverse witness at any hearing in which the legality of such seizure may properly be raised.