Chris Brown, a double GRAMMY nominee and scheduled performer, has been investigated for allegedly assaulting longtime girlfriend Rihanna on Saturday night. According to the police report it said he and a woman were driving in the ritzy neighborhood of Hancock Park when they began to argue around 12:30 a.m. Sunday. Brown stopped the car and both got out, whereupon the argument escalated, the report said. Rihanna, who had visible injuries when police arrived, identified Brown as her attacker, but he had left the scene, the report said. Brown turned himself in to authorities and was briefly held before posting $50,000 bail Sunday night, jail records showed. Brown and Barbados born beauty Rihanna, also nominated, were both slated to perform in different slots at the GRAMMY awards, but separately dropped out at the last minute. Brown and Rihanna were seen together earlier that evening at Clive Davis' pre-Grammy party and later on at BLVD 3 a nightclub in LA. It still isn't immediately clear whether Rihanna's absence was related to Brown's, but Rihanna was admitted into Cedar Sinai's ER after suffering visible injuries to her face. Police booked the 19-year-old R&B singer and dancer on suspicion of making a criminal threat, a felony. Police did not explain how their initial report of an injury assault related to the charge, but it will ultimately be up to the district attorney's office to decide what charges, if any, should be brought against Brown. The Grammy telecast was filled with eye-popping performances, but it was hard to quickly patch up the gaping hole in the telecast caused by the absences of Brown and Rihanna. She was supposed to sing "Live Your Life/Disturbia" as the second performance of the night, he was later to sing "Forever."