The sealed opinion of Will County Judge Stephen White bars eight of the 13 hearsay statements that prosecutors need for the trial of the former Bolingbrook police officer, the Arlington Heights (Ill.) Daily Herald reports.
Peterson has been charged with killing his third wife, Kathleen Savio, whose 2004 death was originally ruled accidental.
His fourth wife, Stacy, who disappeared in October 2007, is still missing.
After presiding over a lengthy hearsay hearing, Judge White issued a four-page sealed ruling in May obtained by the Daily Herald.
White ruled prosecutors proved Peterson killed both women "by a preponderance of the evidence," but nearly all statements attributed to Stacy Peterson "do not provide sufficient safeguards of reliability."
The statements are crucial to the prosecution's case since it lacks other evidence.