Rihanna: "He left me unconscious".

Rihanna is now cooperating with the investigators about Chris Brown's behavior that resulted to physical abuse to her. E! reports that Brown choked her while threatening to kill her and, subsequently, she lost consciousness.

This is the situation. 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 Hancock 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.

What kind of reason that triggered this abusive act? It seems Brown's mind got blackout that you will live a victim unconscious.