Chris Brown is being investigated over an alleged assault in a West Hollywood car park.
US police responding to a report of six men fighting on Sunday night found the scene clear, but were told by witnesses that there had been a brief fight over a parking space.
"The altercation allegedly led to Chris Brown punching the victim," Los Angeles County's Sheriff's department said.
Celebrity website TMZ - which first reported the fight outside the Westlake Recording Studio - said it also involved Frank Ocean, one of the top nominees at Grammy Awards next month.
In a Twitter posting later, Ocean alleged he "got jumped by (Brown) and a couple guys" and suffered a cut finger.
No arrests were made. Singer Brown was gone by the time deputies arrived but the department said the investigation is continuing and he would be contacted.
It is not Brown's first problem in the run-up to the Grammys. His attack on singer Rihanna on the eve of the 2009 awards event overshadowed the show.
Last June, he was injured in a brawl with members of hip-hop star Drake's entourage at a New York nightclub.