Solange Knowles was allegedly "drunk" when she attacked Jay Z in an elevator.
The 29-year-old star, who is the younger sister of Jay's superstar wife Beyonce, had been dealing with problems in both her career and personal life in the time leading up to her outburst in a lift at the Standard Hotel in New York on May 5.
A source told the New York Post newspaper: "Clearly she was on something at the party, she was drunk and out of it. She got in the elevator with Jay and Bey. He had made a comment that she didn't like, and she just snapped."
Another onlooker added: "She was the girl who drank too much and was behaving badly at the party."
Solange has reportedly been dealing with various issues of late, including her complicated music career.
It's thought the star was either dropped by or left Jay's Roc Nation record label - which managed her DJing career - last year, and she has since launched her own company, Saint Records, which has been a strain on her.
An insider explained: "She used to be on [Jay Z's] label, but now she isn't. And she moved to New Orleans from Brooklyn with her son. Solange still spends a lot of time in New York, but she is further away from her family, which has created some issues."
Shocking footage emerged earlier this week of Solange kicking and slapping Jay, 44, inside an elevator following a Met Gala after party. Beyonce was also in the lift but wasn't involved in the affray.