Alyson tweeted about walking into the party behind Tom and Mark and witnessing the whole thing, which she initially thought was a joke ... and she claims Probst tried to break it up!
Apparently, Jeff was speaking for the tribe.
Patton and Billy were definitely there ... 'cause they posed for pics with Tom after the fight.
Tom was Bryan's plus-1 ... and Arnold claims the Oscar-winning filmmaker has vid of the whole thing. And, here's what's revealing. Law enforcement sources tell TMZ Arnold did not give cops the video. An eyewitness says it was weird Bryan was shooting the whole time, and it may be the video was for Tom's show and he doesn't want to get scooped by the police if they release it.
One thing's for sure ... the lead detective will have to suppress any desires to ask for autographs.