Detectives are investigating an alleged $15 million mediation deal between Mel Gibson and ex-gf Oksana Grigorieva.

The Los Angeles County Sheriff’s Department is investigating the matter in relation to both the actor’s extortion claim against the Russian singer and her domestic violence claims against the star, according to gossip website TMZ.

It is believed Grigorieva promised not to release explosive audio tapes documenting their vicious arguments and alleged attacks made by the Hollywood-land star as part of the mediation agreement.

The $15 million deal – which the singer ultimately rejected – included child support, a house and other assets in return for keeping all “evidence” associated with the case confidential.

A source connected with the mediation said: “Oskana and her lawyer pulled out all the ammo they had that would embarrass Mel, and now things are surfacing after the fact that call her credibility into question.”

The Russian claims to have photographic evidence of a black eye following a January 6 fight with the 54-year-old actor.

The close-up shot allegedly shows the 40-year-old with deep bruises around her left eye.

Sources involved in the mediation say she never mentioned a black eye or the photos during discussions.

Sheriff’s investigators are looking at all the claims and evidence to evaluate her credibility and to determine if there is proof of extortion.

Meanwhile, Grigorieva has appointed a new public relations manager Howard Bragman after her last publicist Steve Jaffe quit on Wednesday (28.07.10).

Hmm….interesting Mr Bond!