ST. LOUIS (AP) — Attorneys defending Missouri Gov. Eric Greitens in a criminal case used a highly unusual tactic when they sought to the call the prosecutor as a witness as they attacked her handling of the case.

And the judge made another highly unusual move when he took a step toward having the prosecutor testify during Greitens' trial on a felony invasion-of-privacy charge stemming from allegations that he took a compromising photo of an ex-lover without her permission.

The prosecutor dropped the case, though her office said it could be refiled or pursued by a special prosecutor.

The legal twists occurred because the first-term Republican governor's attorneys made their allegations of misconduct against St. Louis Circuit Attorney Kim Gardner and an investigator a key part of their defense. And St. Louis police announced Tuesday that they would investigate the handling of the case by Gardner's office.

Ethics rules tell lawyers that they can't litigate cases in which they're witnesses, but such conflicts are uncommon in criminal cases.

"It's extraordinarily rare," said Jean Paul Bradshaw, a Kansas City attorney and a former U.S. attorney for western Missouri. "But this case is full of things that are extraordinarily rare."

Criminal defense attorneys don't routinely list prosecutors as witnesses in criminal cases to pressure them into dropping charges because judges generally wouldn't allow it, several law professors said. A prosecutor would have to have firsthand, relevant knowledge of a crime to be called as a witness, they said.

"You can't invade the mind of the prosecutor if there are other ways to get the evidence," said John Ammann, a Saint Louis University law professor.

But Greitens' attorneys have attacked Gardner's handling of the invasion-of-privacy case against the governor since a grand jury indicted him in February. Greitens acknowledged having an affair with his former hairdresser in 2015, which was before he was elected governor, but has denied criminal wrongdoing.

Gardner hired former FBI agent William Tisaby as a private investigator. Greitens' attorneys have accused Tisaby of perjury and Gardner's office of withholding evidence favorable to the governor.

Tisaby declined last month to answer questions for attorneys, invoking his right to avoid incriminating himself. His attorney, Jermaine Wooten, said Tuesday that Tisaby did not have time to prepare for questioning.

"He's very upset he's being used as a scapegoat to this matter," Wooten said. "He's an honest and decent man."

Ben Trachtenberg, an associate University of Missouri law professor, said Tisaby's refusal to answer questions made it "more plausible" that Gardner would be called as a witness.

The circuit judge presiding over the case, Rex Burlison, met privately with attorneys Monday in his chambers about whether Gardner should be called as a witness.

A partial, unsealed transcript showed Burlison told the attorneys that while he was allowing Greitens' lawyers to list Gardner as a witness, he as the judge still needed to determine "whether that witness can actually be used."

He said he faced the choice of denying the defense a witness it felt would help its case or "somewhat blindly" allowing Gardner to testify without knowing whether the evidence's value would outweigh the potential negative effects on the state's case. Or, he said, he could allow her to be questioned privately to examine those issues — the course he ultimately took.

Robert Dierker, an attorney in Gardner's office, said that if Tisaby were subpoenaed as a witness for the trial and refused to answer questions, her office would seek immunity for him so that Tisaby could be compelled to testify. Greitens' attorneys questioned having immunity granted by an official they have accused of misconduct.

The transcript shows that Burlison scheduled a private two-hour session for questioning Gardner so that he could determine whether she would be a witness. Dierker had told the judge that Gardner would take "an active role" in presenting the case at trial.

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Hanna reported from Topeka, Kansas.