Los Angeles  - As among the brightest and most successful young stars in the US music industry, Chris Brown and Rihanna were a match made in publicists' heaven. But just as they were set to send their relationship into hyperdrive with a joint appearance at the Grammy Awards on Sunday night, the fairy-tale romance turned sour and very ugly.

Brown, just 19, stands accused of making criminal threats against Rihanna, 20, and police say that prosecutors could charge him with more - or less - depending on their investigation.

Though initial reports about what happened were murky, the facts were beginning to emerge Monday. The couple had been staying the weekend at a luxury suite in the trendy London Hotel. Brown had rented a silver Lamborghini for the weekend's celebrations and the two were seen canoodling at the pre-Grammy gala held Saturday night at the Beverley Hilton.

But on the way home something went awfully wrong. The pair were seen engaged in a furious argument inside the vehicle in the ritzy Hancock Park neighbourhood. Rihanna got out of the car, and Brown followed. Rihanna apparently called the police, but by the time they arrived Brown was gone.

A police report said that the victim showed obvious signs of injury and identified her attacker as Brown. Celebrity website TMZ reported that Rihanna, whose real name is Robin Fenty, had bite marks and contusions. Fox News said she was treated at a hospital and later released.

After the incident, they checked out of their hotel together early Sunday morning and later both cancelled their appearances at the Grammys.

The music industry's biggest night of the year continued without them, but talk of their emerging scandal overshadowed the actual ceremony. Many in the audience were checking celebrity websites throughout the evening to find out what was going on. Ironically, as the record of the year prize was being presented Brown was giving himself up to police, in the company of celebrity lawyer Mark Geragos, and he was released on 50,000 dollars bail.

The couple were first rumoured to be in a relationship at the end of 2007. Though they consistently refused to talk about their romance, they were first photographed together at the 2008 Grammys and offered many signs of their bond, from matching star tattoos on their necks to getting affectionate in public and enjoying romantic vacations in Europe and the Caribbean.

By this time both were firmly established as young superstars. Barbados-born Rihanna released her first hit album A Girl Like Me in 2006, and followed that with the megahit Good Girl Gone Bad, which yielded eight hit singles, including radio standards such as Umbrella, Don't Stop The Music and Take A Bow. Despite her young age she is one of the two female solo artist with the most number ones this decade, with the other being Beyonce.

Brown made his recording debut in 2005 at the age of 16 with a self-titled album that yielded the hit single Run It!, which made him the first male artist to have his debut single top the Billboard charts. His second album Exclusive spawned the worldwide hits Kiss Kiss and With You.

Hollywood insiders said while they were sad at the turn of events they were not too shocked. "Keeping a relationship going is hard as it is," said one female record executive. "In the glare of the publicity spotlight, it's even harder, especially when you're so young and successful."

It can be especially difficult for young male rap stars, the executive said.

"Unfortunately the ethos in that world can be very macho and at times misogynistic," she said. "Rap stars have inflated egos - but Rihanna was probably an even bigger star than Brown so how did he deal with that?" (dpa)