Is This Legal?

I applaud Tom Morgan for his great story and apparent persistence in investigating the senseless death of his sister, but how can he accuse these frat guys of the beating/murder and subsequent arson without any proof? And how did he arrive at these conclusions? It doesn't seem legal or moral to make a movie about real life characters depicted as murderers/arsonists unless you have actual proof and this movie didn't show any of that.

I'm surprised he was able to get away with making this film without several lawsuits from the frat guys and even Mr. Swenson.