TMZ has learned ... L.A. County Sheriff's investigators have just interviewed the lawyer who mediated Oksana Grigorieva's case with Mel Gibson, in connection with the extortion investigation.
Law enforcement sources tell TMZ ... two investigators went to the law offices of Eric George, a well-known L.A. attorney. As TMZ has reported, the Sheriff's investigators are looking into whether Oksana tried to extort $15 million from Mel Gibson, in return for the infamous audio tapes. TMZ has already posted the signed agreement that Oksana later disavowed.
Our law enforcement sources say ... among the topics investigators covered:
-- How involved Oksana was in the mediation -- How involved she was in the money discussions -- Why she ultimately rejected the agreement she signed
In addition to meeting with George, investigators met with his law partner, Sonia Lee, who was also involved in the mediation.
Our law enforcement sources say the extortion file should go to the L.A. County District Attorney shortly. The D.A. will make simultaneous decisions on whether to prosecute Mel for domestic violence and Oksana for extortion.