The US Supreme Court refused Monday to hear an appeal on behalf of an Illinois eavesdropping law that prohibits recording a law enforcement officer, leaving in place a federal appeals court ruling that barred enforcement of the law on the grounds it “likely violates” free speech guarantees.

The eavesdropping law, which makes recording law enforcement officers a first-class felony punishable by up to 15 years in prison, is among the most severe in the nation regarding surreptitious audio or video recording of public or private conversations.

Illinois is one of just 10 states, plus the District of Columbia, where all parties must give their consent to a recording, according to data provided by The National Conference of State Legislatures.

The Supreme Court ruling is a major setback for Cook County State Attorney Anita Alvarez, who has waged a rigorous campaign against those who have violated the Illinois law.

Ms. Alavarez had appealed the May ruling by the US Court of Appeals for the Seventh Circuit, which barred her office from prosecuting staffers of the Illinois chapter of the American Civil Liberties Union for monitoring police officers’ public actions.

“The Illinois eavesdropping statute restricts far more speech than necessary to protect legitimate privacy interests,” the appellate court said in its ruling, “it likely violates the First Amendment’s free speech and free press guarantees.”

That decision arrived just before the NATO summit in Chicago when thousands of protesters were expected to swarm the city’s downtown area, most armed with some kind of digital recording devices. The city said it would not enforce the law during that time.

Lead public attorneys in other Illinois counties have been hesitant to enforce the law as written, saying either that the punishment is too excessive or that it presents constitutional flaws.

Alvarez, however, has made headlines over the past two years for aggressively enforcing the law. The most high profile case is the 20-month jailing of Annabel Melongo for recording phone conversations with a county clerk before a judge released her in October 2011 amid public scrutiny of Cook County’s measures.

Another woman, Tiawanda Moore, was acquitted in August 2011 after being charged with recording Chicago police investigators who she felt were trying to dissuade her from filing a sexual harassment complaint against a fellow officer.

David Crane, a constitutional law expert at Syracuse University of Law in New York, says he is not surprised the high court rejected the provision involving law enforcement because the court typically “errs on the side of constitutional rights and standard” unless the state or federal governments “show compelling interests to supersede.”

Rapid advances in recording techniques – such as the implementation of video and sound recorders on mobile phones – has forced the court to re-examine eavesdropping laws every few years.

“The Supreme Court pays real close attention to the parameters [of free speech rights] and there’s always real tension when the law falls behind technology advances,” Mr. Crane says.

Emboldened by the high court action Monday, the ACLU in Illinois said it plans to pursue a permanent injunction against the law enforcement provision and want to expand its efforts to allow the public monitoring of public officials without threat of prosecution.

In a statement released late Monday, the Cook County State Attorney’s office said, “we respect and accept the Court’s decision in this matter and we are continuing to review all legal options as the [ACLU] case proceeds in U.S. District Court.”

Harvey Grossman, legal director for the ACLU of Illinois, says, meanwhile, that “while a final ruling in this case will only address the work … to monitor police activity, we believe that it will have a ripple effect throughout the entire state.”

“We are hopeful that we are moving closer to a day when no one in Illinois will risk prosecution when they audio record public officials performing their duties,” he says. “Empowering individuals and organizations in this fashion will ensure additional transparency and oversight of public officials across the state.”