This Chicago Sun-Times article reports:
A Cook County judge today threw out an indictment against a Chicago Police officer accused of falsifying drunken driving arrests.
Judge James M. Obbish said prosecutors were wrong to rely on statements Officer John Haleas gave to police investigators during an internal investigation of his conduct.
Under union rules, such statements cannot be used against an officer in a criminal case, the judge said, ruling on a defense motion to dismiss the case….
I don’t have access to the judge’s opinion — if any of you can point me to it, I’d love to see it — but I highly doubt that union rules (even in Chicago) can affect the admissibility of evidence in criminal trials.
But the Supreme Court’s caselaw on the privilege against self-incrimination, and in particular Garrity v. New Jersey might well be doing the work here:
Appellants were police officers in certain New Jersey boroughs. The Supreme Court of New Jersey ordered that alleged irregularities in handling cases in the municipal courts of those boroughs be investigated by the Attorney General, invested him with broad powers of inquiry and investigation, and directed him to make a report to the court. The matters investigated concerned alleged fixing of traffic tickets.
Before being questioned, each appellant was warned (1) that anything he said might be used against him in any state criminal proceeding; (2) that he had the privilege to refuse to answer if the disclosure would tend to incriminate him; but (3) that if he refused to answer he would be subject to removal from office.
Appellants answered the questions. No immunity was granted, as there is no immunity statute applicable in these circumstances. Over their objections, some of the answers given were used in subsequent prosecutions for conspiracy to obstruct the administration of the traffic laws. Appellants were convicted and their convictions were sustained over their protests that their statements were coerced, by reason of the fact that, if they refused to answer, they could lose their positions with the police department….
The choice given petitioners was either to forfeit their jobs or to incriminate themselves. The option to lose their means of livelihood or to pay the penalty of self-incrimination is the antithesis of free choice to speak out or to remain silent. That practice, like interrogation practices we reviewed in Miranda v. Arizona, is “likely to exert such pressure upon an individual as to disable him from making a free and rational choice.” We think the statements were infected by the coercion inherent in this scheme of questioning and cannot be sustained as voluntary under our prior decisions….
Perhaps there are union contract provisions that reflect the Garrity principle. But the rule of exclusion of evidence would be set by Garrity or perhaps by any Illinois legal rules that might be based on Garrity, not by the union contract itself.
UPDATE: I forgot to make this clear at first: I think that the judge probably didn’t rely on union rules, but instead relied on Garrity or cases or statutes that flow from it.