*UPDATE* Full Fight Video Added. Possible Reason For Altercation Revealed - Jay Z Ferociosly Attacked By Beyonce's Sister Solange (Video)

via TMZ

Jay Z was ferociously assaulted by Beyonce's sister Solange  ... who was wildly kicking and swinging at him inside an elevator ... and the attack was captured on surveillance video ... obtained by TMZ.

According to TMZ, it all went down at a Met Gala after party last week at the Standard Hotel in NYC.  In the video ... Jay Z, Beyonce and Solange step into the elevator ... and then Solange goes crazy, screaming at Jay before unleashing a violent attack. 

A large man -- who appears to be a bodyguard -- attempts to hold Solange back, but she manages to connect at least 3 times.  At one point Solange throws a kick and Jay grabs her foot, but never attempts to strike her.  Beyonce stands by without getting physically involved. 

It's unclear what triggered the fight.

And this is interesting.  When you look closely, you see the bodyguard hit the emergency stop switch on the 12th floor -- mid fight -- presumably to keep the fight private.

All three walked out of the building together -- Solange looks pissed -- but while Bey and Solange get in one car ... security walks Jay to a different one. 


Watch Video of Attack HERE

**UPDATE**

TMZ has added the full fight video between Solange and Jay Z. One thing is clear, Solange is in an uncontrollable rage and her sister Beyonce doesn't seem to want any part of it.

While nobody knows for sure why the singer is so mad at her brother-in-law, US Weekly is reporting that it has to do with something Jay said.

According to their source: 

"Solange flipped out over something Jay had said, that she took the wrong way. She took it too hard or too far. Not sure what it was, but she greatly overreacted."

Full Fight Video HERE