In the past, I’ve seen investigation style plots fail, fail, and…fail. Often, it seems a big factor is that the players don’t know what questions to ask. Sometimes they (as players, not characters) don’t care to do any investigating. Maybe this strategy would help, it’s a brainstorm.
What if the GM made a list of questions that needed to be answered, and assigned XP (or other reward) to each of them? The list could then be distributed to the players, so that they know what direction they need to head in. Of course, it would have to be worded in such a way that it didn’t give anything away, but it could always be added to once the players find out more information. I suspect it would also encourage more people to participate in investigating.