PONTIAC (AP) - No charges will be brought in an alleged plot to blackmail Michigan's attorney general for having an extramarital affair, closing a chapter in the state's most bizarre political story in recent years.

Oakland County Prosecutor David Gorcyca said yesterday he did not have sufficient evidence to prove that attorney Geoffrey Fieger - through lawyer Lee O'Brien - threatened to expose Mike Cox's affair. Gorcyca said a conspiracy probably did take place, and he plans to submit the matter to the Attorney Grievance Commission.

"Neither Mr. Fieger or Mr. O'Brien should claim victory, act virtuous or gloat," Gorcyca said. "Far from it. In my opinion - and based upon my review of the fact - the evidence soundly convinces me that a severe and reprehensible ethical violation or violations were committed by both Mr. Fieger and Mr. O'Brien."

Cox admitted cheating on his wife last week and accused a Fieger associate of trying to expose the indiscretion unless he stopped investigating Fieger for possible campaign finance violations. The attorney general said he respected Gorcyca's decision.

"It was his call to make," Cox told The Associated Press. "He's 100 percent confident these guys worked together to stop an ongoing investigation using threats. - Regular folks understand that proof beyond a reasonable doubt is not always reachable. That doesn't mean something never happened."

Fieger immediately called for Cox's resignation, saying his allegations were "evil and malicious" and adding there was never a "scintilla" of evidence that he committed a crime.

"You have brought scorn and disrespect upon your office," Fieger said. "For the welfare of the state of Michigan you must resign."

Cox, a Republican, said he plans to run for re-election next year. Fieger, a Democrat who ran unsuccessfully for governor in 1998 and became famous for defending assisted suicide advocate Jack Kevorkian, has been highly critical of Cox and last month announced he wants to run against him for attorney general.

Elbert Hatchett, O'Brien's attorney, criticized Gorcyca for his "editorial commentary" about Fieger and his client.

"You either prosecute the case or you don't," Hatchett said. "You don't get on television and make excuses that involve disparaging the character of people in the investigation and then walk away without charges. It's very irresponsible."

Cox is investigating Fieger's role in a $457,000 television ad campaign that urged viewers to vote against Justice Stephen Markman in last year's state Supreme Court election. Fieger did not file papers acknowledging he paid for the anti-Markman ads until seven months after the election, and county clerks say the disclosure should have been made much earlier.

Cox said O'Brien contacted his office on Oct. 14 and relayed Fieger's threats to expose his affair. O'Brien subsequently met with a Cox staff member and Fieger at a Morton's steakhouse in Southfield, Gorcyca said.

"While no definitive statements or legally admissible extortion threats were stated at the meeting, I am 100 percent confident there was a meeting of the minds and each was very cognizant of the quid pro quo agreements being discussed," Gorcyca said during a news conference at his office.