9:06 AM PT -- TMZ has obtained the 911 call placed from the Airbnb the night of the shooting at 11:43 PM on December 31. The frantic caller said an argument broke out and then "they just started shooting."
He added he did not see who the shooter was but, when asked if anyone is bleeding, he says, "They're both dead."
Olympic gymnast Simone Biles' older brother is in police custody for the murders of 3 people ... after an 8-month investigation into a New Year's Eve tragedy.