OKLAHOMA CITY (AP) — An Oklahoma Highway Patrol captain accused of using cellphone audio recordings to try to blackmail the commissioner of the state Department of Public Safety and further his career has been charged with extortion.

Attorney Mike Johnson said Capt. Troy D. German "100 percent" denies the accusations, calling his client "a whistleblower who exposed corruption at the highest levels at the highway patrol."

A grand jury charged German on Thursday with extorting Commissioner Rusty Rhoades to gain a promotion or appointment to a state office, The Oklahoman reported.

According to a court affidavit, Rhoades told Highway Patrol investigators that he and German had three meetings during which German made verbal and written attempts to extort him. Rhoades said German alleged he has six audio recordings on his personal iPhone that would incriminate the commissioner and two other Highway Patrol officials.

Rhoades told investigators that German sought his backing for his appointment to director of the Oklahoma Department of Emergency Management, the affidavit says. He said German threatened to release evidence of Highway Patrol misconduct if the commissioner rejected his proposals.

The Highway Patrol completed search warrants of German's iPhone and Apple user ID. The grand jury assessed the evidence in the case before returning with an indictment.

Johnson said German has overwhelming proof that he was attempting to transform the culture at the Highway Patrol in which "the good ol' boys were making sure their friends got promoted."

"We look forward to presenting all of our evidence in front of an impartial jury because I doubt seriously that all the evidence was presented to the grand jury," he said.

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This story has been corrected to show German is a captain, not the chief.

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Information from: The Oklahoman, http://www.newsok.com