The Russian singer is the mother of Mel Gibson's eighth child.
Sheriff's detectives in Los Angeles are checking extortion allegations against Mel Gibson's ex-girlfriend.
Sheriff's spokesman Steve Whitmore says the agency is looking into whether Russian singer Oksana Grigorieva may have tried to extort the Academy Award-winner. He declined to offer any other details on the inquiry.
Detectives earlier this month interviewed Grigorieva after she claimed Gibson abused her during an incident in January.
A spokesman for Grigorieva declined comment Wednesday.
Gibson and Grigorieva are locked in a bitter custody dispute over their infant daughter.
No arrests have been made, and neither has been charged with any crimes.