Oksana's Credibility Scrutinized by Sheriff

1/26/2011 2:00 PM PST

Sheriff's Department Asks Judge for Court File to Evaluate Oksana Grigorieva's Credibility in Mel Gibson Domestic Violence Investigation

The L.A. County Sheriff's Department is going into court this AM ... asking the judge in the Mel Gibson/Oksana Grigorieva custody war to turn over court files to see if Oksana has told conflicting stories about alleged domestic violence ... TMZ has learned.

Lawyers for the Sheriff's Dept. just arrived in court.  They're asking for a copy of the court file to evaluate the domestic violence case ... but we're told the real focus is everything Oksana has said about events surrounding the blowout on January 6, 2010.  We're told investigators want to look at Oksana's declaration and deposition testimony in particular, to determine if she's changed her story.

Based on documents previously reviewed by TMZ and various other information, investigators will find inconsistencies.

We're told the latest request could have a bearing on whether domestic violence charges are filed against Mel and whether the D.A. will file extortion charges against Oksana.

UPDATE:  Sources say Judge Scott Gordon just approved the Sheriff's request for the documents.