By Dan Whitcomb
LOS ANGELES (Reuters) - Singer Chris Brown was under investigation by police for attacking an unidentified woman after an argument early on Sunday, and hours later both he and girlfriend Rihanna missed their performances at the Grammy Awards.
All About 2009 Grammys
Grammys 2009 Rewind
Our Grammy 2009 predictions
Grammy Nominees
Todo sobre los Grammys 2009
Relive the Latin Grammys
Terra Newsletter: Get the news in your inbox
Música en español
In Headphones: Our music blog
The best videos on TerraTV
Awesome music on Terra Radio

Police declined to name the woman who made the domestic violence complaint.
Rihanna had been scheduled to perform her hit single "Disturbia" at the Grammys, but canceled shortly before the start of the show, the National Academy of Recording Arts and Sciences said in a statement.
"We have just been informed that Rihanna will not be attending tonight's 51st Annual Grammy Awards. We're sorry she is unable to join us," the academy said. There was no further explanation for her absence.
Soul legend Al Green performed alongside pop star Justin Timberlake in Rihanna's place.
Grammy officials made no mention of Brown's performance but he did not appear on stage as scheduled.
Neither Rihanna, who was nominated in three categories, nor Brown, who was nominated twice, walked the red carpet, and they were not in attendance for the awards ceremony. Their representatives could not be reached for comment.
Police said Brown got into an argument with an unidentified woman while sitting in a parked car in the city's Hancock Park neighborhood at around 12:30 a.m. on Sunday.
"After stopping his car, Brown and the woman got out and the argument escalated," the Los Angeles Police Department said. "The woman suffered visible injuries and identified Brown as her attacker."
The LAPD received a 911 call reporting the disturbance and according to the statement, when officers arrived they found the victim, but Brown had already left the scene.
Brown, who broke onto the Billboard charts at the age of 16 with his hit single "Run It" and sold 2 million copies of his self-titled debut album, also scored a pair of hits on his sophomore effort, "Exclusive," in 2007.
Barbados-born Rihanna also released her debut album in 2005 and had a number one hit single with "SOS" less than a year later. She followed with three more number ones: "Umbrella," "Take a Bow" and "Disturbia."
She won a 2007 Grammy Award for her single "Umbrella," a collaboration with the rapper Jay-Z.
Rihanna y David Bisbal - Hate that I Love You
Terra/Reuters