Only thing I can think of is underage drinking or illicit drugs, and must have been afraid to call.That is one question that all will keep asking, I don't get it, you hear noise, see a stranger in mask, and why no immediate call to the police. I guess at the time it would be hard to even imagine what had actually happened, still the lack of immediate action by the roommates is hard to understand.
It still doesn't make any sense.