The disclosure came after 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.
Gibson and Grigorieva have been involved in a nasty custody dispute over their infant daughter - the subject of a confidential court case.
Whitmore declined to disclose further details of the domestic violence investigation or any potential evidence.
"We're in the process of verifying, verifying, verifying," Whitmore 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 breakup with Grigorieva, 40. The former couple first revealed their relationship shortly after Gibson's wife, Robyn, filed for divorce last April. The divorce is pending.
Court proceedings in the case involving the 8-month-old daughter of Gibson and Grigorieva have been conducted in closed session.
Gibson's attorney, Stephen Kolodny, previously said the actor has paid Grigorieva "tens of thousands of dollars" and provided her with a home, vehicle and nanny since their breakup.
He also accused the singer of violating the terms of a confidential custody agreement reached in May.
Grigorieva has said she cannot discuss the case.
Attorneys handling the matter for Gibson and Grigorieva did not respond to e-mails seeking comment Thursday.
The Sheriff's Department said the alleged attack on Grigorieva occurred Jan. 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 drunken driving. The station is located in suburban Agoura and also covers Malibu, where Gibson lives.
Gibson eventually apologized 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 drunken driving case and his conviction has been expunged.
Gibson, 54, won an Academy Award 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.