Surveillance video from the moment -- published by TMZ -- finds Solange kicking and swinging at her sister's husband after she joins Jay Z and Beyonce in an elevator. Solange is soon held by Jay Z's security guard after the attack, but she continues to thrash at the rapper.
While it's still not clear what caused the argument, TMZ points out that the third man stopped the elevator at one point as if trying to keep the altercation private.
Another clip posted by TMZ finds the trio exiting the party together, but splitting ways before entering their vehicles as Solange and Beyonce jump into a car together and Jay Z walks up to a different one.
Neither Jay Z, Beyonce or Solange have addressed the incident thus far.