The Russian singer was interviewed earlier this month by detectives after she claimed Gibson abused her in January.
The two are in the middle of a bitter custody dispute over their 8-month-old daughter, Lucia.
RadarOnline.com reports that the Los Angeles County Department of Children and Family Services investigated Mel Gibson for several hours on Wednesday.
The website also obtained a photo which allegedly shows the couple's baby with a mark on her chin, a mark Grigorieva claims was evident after Gibson punched her. Gibson's lawyers are accusing the star's ex-girlfriend of trying to extort money from the actor.
"Given that Mel's camp have leveled these allegations, as in any particular case, law enforcement officials are duty-bound to investigate, and they've indicated today that's what they'll do in regards to the claims of extortion," said Dylan Howard, senior executive editor of RadarOnline.com.
No arrests have been made, and neither has been charged with any crimes.
The Associated Press contributed to this report.