The mention of a polygraph test led to a mistrial yesterday in the case of a Junior ROTC instructor accused of sexual misconduct with two teens.

Jackson police Chief Len Keller testified that he interviewed Kevin B. Johnson at Seneca Valley Senior High School in January 2008 about the allegations against him. Fifteen minutes into his testimony yesterday, the chief said he had raised the issue of a polygraph test during the interview, and that Mr. Johnson had declined it.

David S. Shrager, Mr. Johnson's attorney, objected to the mention of a polygraph test -- which isn't admissible in court -- and asked Butler County Judge Timothy F. McCune to declare a mistrial. Judge McCune granted his request.

"This statement would permeate all the further information that would come in this trial," the judge said.

Before his testimony was halted, Chief Keller said the investigation involving Mr. Johnson began Jan. 10, 2008, when police were notified that a male student claimed the Junior ROTC instructor had made sexual advances toward him. They interviewed the student and his mother, then identified another male student they believed had close contact with Mr. Johnson.

On that January afternoon, they interviewed Mr. Johnson for about 10 minutes, Chief Keller said. The defendant told police he had a "detailed relationship" with one of the male students, but declined to elaborate, and then refused a polygraph.

In his opening statement, Mark Lope, the assistant district attorney for Butler County, said the two alleged victims intended to testify. The Pittsburgh Post-Gazette does not identify alleged victims in sexual assault cases.

One of the students claimed that Mr. Johnson made sexual advances, which he rebuffed, on five separate occasions between October 2006 and July 2007. The other student claims he had sexual encounters with Mr. Johnson starting in spring of 2005, his freshman year, until Jan. 4, 2008.

Bruce Antkowiak, a law professor at Duquesne University, said there are no circumstances in a criminal trial when a polygraph test or its results can be referenced. The courts prohibit mention of polygraphs for the same reason they prohibit calling body language or voice inflection experts to testify whether they think the defendant is being truthful, he said.

"The court thinks it usurps the fundamental role of the jury to assess the person's credibility, and it is an area on which they may not tread," Mr. Antkowiak said.

No date has been set for a new trial.