LANSING, Mich. (AP) - Attorney General Mike Cox acknowledged yesterday having an affair a number of years ago and accused an associate of trial lawyer Geoffrey Fieger of threatening to expose the indiscretion.

Fieger, who won the 1998 Democratic nomination for governor after becoming famous as the attorney for assisted suicide advocate Jack Kevorkian, has said he wants to challenge Cox, a Republican, in the 2006 attorney general election.

Cox said he came clean publicly because a Fieger associate threatened to expose the affair if Cox didn't stop investigating Fieger. Fieger said the person Cox is accusing is not an associate of his.

Cox said he told his wife Laura about the affair on March 22, 2003. The couple went to counseling and attended a church program, and the affair is now "old news" in his family, he said.

"What I did was inexcusable," Cox said in prepared comments. "I am completely responsible for what happened - it was entirely my fault."

Since March, Cox's office has been investigating a $450,000 television ad campaign that urged viewers to "vote no" against Justice Stephen Markman in last year's state Supreme Court race. Markman easily won re-election. But it was not until June - seven months after the election - that Fieger, a multimillionaire, filed papers in Oakland County acknowledging he paid for the anti-Markman ads.

Fieger said Cox's allegations yesterday were "amazingly sick."

"Something sounds absolutely insane about what's going on here," Fieger told The Associated Press in a telephone interview. "How could the attorney general have so many skeletons in his closet that he could be blackmailed? Why is he subjecting his wife to this humiliation?"

Cox said a lawyer named Lee O'Brien contacted his office Oct. 14. O'Brien told a Cox staff member that "Fieger wants me to deliver a threat to your boss," according to Cox.

A message seeking comment was left on O'Brien's cell phone yesterday afternoon.

Cox said he contacted the Oakland County sheriff and prosecutor, who monitored meetings between O'Brien and his staff member in which O'Brien said Fieger wouldn't do anything if the campaign finance investigation went away. One meeting involved Fieger himself, Cox said.

"They thought I would cower," Cox said. "They thought that my wife did not know nor did they count on my resolve to do my job at any cost. And they did not count on my wife's resolve that justice be done even if it means personal embarrassment."

Oakland County Prosecutor David Gorcyca said Cox contacted him about the case roughly a month ago.

Law enforcement officials turned in material related to their investigation this week, Gorcyca said, and his office is reviewing the material to determine if extortion or obstruction of justice charges are warranted.