LAPD Contacting Celebs Who Witnessed Fight Between Tom Arnold, Mark Burnett

The LAPD has a Hollywood mystery on its hands ... but while the list of central characters reads like a movie cast, the investigation into Tom Arnold's fight with Mark Burnett is 100 percent real.

Celebs who saw the pre-Emmy altercation between Tom and mega-producer Mark can expect a call from the police ... law enforcement sources tell TMZ.

Kevin Bacon, Billy Eichner, Patton Oswalt, Alyson Hannigan, Jeff Probst and Bryan Fogel are among those who allegedly saw Tom and Mark get physical.

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.