The Los Angeles Sheriff’s Department is investigating claims that Oksana Grigorieva extorted ex-lover Mel Gibson AND lied to officials. But Oksana’s lawyers are adamantly saying she did NOT extort Mel!

Even though Oksana Grigorieva managed to get ex-lover Mel Gibson’s horrifying rants on tape and thus provide evidence for a criminal investigation, her execution may have been flawed. The Los Angeles Sheriff’s Department says it is now investigating claims from Mel’s camp that Oksana tried to extort the Braveheart actor for millions of dollars in order to keep the tapes secret. But Oksana’s attorney is coming out swinging against the claims that his client tried to extort Mel.

“Extortion allegations have been brought to our attention and we are certainly going to be looking at that,” LASD spokesperson Steve Whitmore said July 21.

But Oksana’s lawyer Stephen Jaffe tells TMZ that all of these allegations are completely untrue! “My client, Ms. Grigorieva, has repeatedly stated that there is no credibly evidence whatsoever of extortion by her, and she stands steadfastly by that statement.”

Sources close to the case say that Mel’s lawyers have hard evidence against Oksana that will prove said claims, including a text message she sent to the Australian actor about the tapes leaked onto RadarOnline. “You broke your agreement with me,” one message says.

Even if Oksana is charged with extortion, Mel isn’t off the hook from his criminal and civil investigations, says Los Angeles-based criminal defense attorney Steve Cron.

“Prosecution can still go after him, even if she’s extorting him,” Cron tells HollywoodLife.com. “The trouble is whether or not she tape-recorded him legally or not. It’s against the law to tape-record somebody without their consent, unless it’s for the purpose to document the crime.”

While the Sheriff’s Department begins their investigation on Oksana, the Los Angeles County Department of Children and Family Service will continue its investigation to make sure Oksana and Mel are fit parents for their eight-month-old baby girl, Lucia.

–Kirstin Benson