Rihanna arrives at the Recording Academy's Clive Davis pre-Grammy party in Beverly Hills, California February 7, 2009. [Agencies]

Rihanna has reportedly claimed Chris Brown choked her until she was unconscious during an alleged assault on Sunday (08.02.09).

The 'Umbrella' singer is fully co-operating with police - who booked Chris on suspicion of making criminal threats - and has also alleged he threatened to kill her during the attack.

Rihanna and Chris started arguing after leaving Clive Davis' pre-Grammy party on Saturday (07.02.09), resulting in Chris stopping his silver Lamborghini in a Los Angeles side street.

Gossip website E! reports Rihanna threw Chris' car keys out the window, sending the 19-year-old singer into a rage.

It is claimed Rihanna lost consciousness and when she came round Chris had fled. Officers who attended the scene were said to be so shocked at her "horrific" injuries - including a split lip, bruised face and bite marks - they drove her straight to Cedars-Sinai Medical Center.

The singer - who is now in hiding - has cancelled her 21st birthday party, which was due to take place on February 20, and part of her tour.

However, her family have assured her fans she is recovering well.

Her grandmother Clara Brathwaite said: "I don't want people to worry. Rihanna is fine and she is doing well."

Yesterday (10.02.09), the Los Angeles district attorney asked police to look for more evidence in their case against Chris. The investigation needs to be carried out before formal charges are filed against the singer.

District Attorney spokeswoman Jane Robinson confirmed: "Prosecutors asked for further investigation. No further comment at this time."

Chris' former step-father has spoken out about the alleged incident, claiming he "wouldn't be surprised" if the singer had assaulted Rihanna.

Donnelle Hawkins - who Chris has accused of abusing his mother, an allegation Donnelle refutes - said: "I wouldn't be surprised. Chris has to take responsibility for his own actions."