Colorado Supreme Court ruled Monday that Weld County authorities violated the Fourth Amendment and privacy rights of suspected illegal immigrants when they used tax returns to potentially build hundreds of identity theft cases against them.

The ruling affirmed a decision by a Weld County district judge who suppressed evidence against one of the defendants. That judge said authorities had no probable cause to search the man's tax returns and that the documents are confidential.

The defendant was one of more than 70 people charged with criminal impersonation and identity theft. Some defendants pleaded guilty and were deported before the district court ruling. Pending the Supreme Court appeal, many had their cases dismissed without prejudice, which gave prosecutors the option of filing cases again if judges had ruled in their favor.

The investigation, dubbed "Operation Numbers Game," marked the first and only time in the U.S. that authorities used tax documents to try to prosecute suspected illegal immigrants.

Prosecutors had said they believed as many as 1,300 immigrants were breaking the law and that they planned to charge more people. The district attorney's office said prosecutors in other states had expressed interest in their method of investigation.

But in March, District Judge James Hartmann Jr. suppressed the evidence against one of the defendants, saying the search warrant was "nothing more than an exploratory search based upon suspicion that some unknown person or persons" committed a crime.

He also said tax records were confidential under federal law and authorities had no right to inspect the documents.