Mel Gibson is being investigated in a possible domestic violence incident involving his ex-girlfriend earlier this year, sheriff's officials said Thursday. The Los Angeles County Sheriff's Department listed the actor-director as a potential suspect in the alleged attack on Russian singer Oksana Grigorieva at an undisclosed location in Malibu. The two have been involved in a nasty custody dispute over their infant daughter - the subject of a confidential court case. The release says the incident occurred Jan. 6 and that the investigation is in a preliminary stage. The actor has not publicly discussed the breakup with Grigorieva, 40.