Actor Mel Gibson is being investigated over a domestic violence incident involving his ex-girlfriend, Russian singer Oksana Grigorieva, earlier this year.

The disclosure came after Los Angeles detectives interviewed Grigorieva about the alleged attack at an undisclosed location in Malibu.

Sheriff's spokesman Steve Whitmore said Grigorieva was the only person interviewed so far in the case.

Braveheart and Lethal Weapon star Gibson, 54, and Grigorieva have been involved in a bitter custody dispute over their baby daughter - the subject of a confidential court case.

Mr Whitmore refused to disclose further details of the domestic violence investigation or any potential evidence.

"We're in the process of verifying, verifying, verifying," he said. "All the allegations that have been put forth will be thoroughly investigated."

A phone message left for Alan Nierob, Gibson's spokesman, was not immediately returned.

The actor has not publicly discussed the break-up with Grigorieva, 40. The former couple first revealed their relationship shortly after Gibson's wife Robyn filed for divorce last April.

Court proceedings in the case involving the eight-month-old daughter of Gibson and Grigorieva have been conducted in closed session.

Gibson's lawyer Stephen Kolodny said previously the actor had paid Grigorieva "tens of thousands of dollars" and provided her with a home, vehicle and nanny since their break-up.

He also accused the singer of breaking the terms of a confidential custody agreement reached in May.

Grigorieva has said she cannot discuss the case.

Lawyers handling the matter for Gibson and Grigorieva did not respond to emails seeking comment.

The sheriff's department said the alleged attack on Grigorieva happened on January 6 and the investigation was in a preliminary stage. It could take a month before the inquiry is concluded and the results are turned over to the district attorney's office, which will decide whether to pursue charges.

The investigation was being conducted by the same sheriff's substation that arrested Gibson in 2006 on suspicion of drink driving. The station is in suburban Agoura and also covers Malibu, where Gibson lives.

Gibson eventually apologised for making a series of anti-Semitic and sexist remarks during his arrest. He also said he had battled alcoholism for all of his adult life.

Gibson is no longer on probation for the drink-driving case and his conviction has been expunged.

Gibson won an Oscar in 1995 for directing Braveheart, which won that year's best picture Oscar.

His remarks in 2006 led to a lengthy cooldown period for the actor, who was once one of Hollywood's most bankable stars.

Edge Of Darkness, released earlier this year, represented Gibson's first starring role since 2002.