They began squabbling after leaving Clive Davis' pre-Grammy party late Saturday. Shortly after midnight, things blew up. Brown pulled his silver Lamborghini to the side of a street in L.A.'s Hanc**k Park neighborhood. That's when, per the source, Rihanna grabbed the car keys and tossed them out the window, sending Brown into a rage.

He tried in vain to find the keys, then came back to the car, put his hands around her neck and, according to the insider, said, "I'm going to kill you!"

According to the source, the 20-year-old "Disturbia" singer told police that she lost consciousness, and when she awoke, Brown had fled.

According to the source, her right eye was blackened and badly swollen and she had hand prints on her arms. The responding officers were so concerned about her, they drove Rihanna to the hospital in their squad car instead of waiting for an ambulance.

The Umbrella singer may need plastic surgery.