It's often fun--and sometimes necessary--to put an investigation scene into an adventure. You're going through three layers to create one:

* Your adventure design, which requires the heroes find out one or several things.
* Your presentation to the GM, who must understand how to get the players to what they need to understand.
* The players, who must find the investigation engaging and useful.

This is not easy! 

It helps to work on these from top to bottom, and I'm going to talk about writing each. First, you have to know where the heroes are going to go after they succeed at the investigation, in "musts" and "mights." 

"Musts" move the narrative forward; until the heroes learn these things, the investigation isn't over. The most common examples of musts are where the heroes need to go next, and what they need to do when they are there. (Examples: the king's evil advisor has the Mindwarping Gem, the king's evil advisor is in the catacombs, the Mindwarping Gem can be destroyed by striking it with a silver hammer)

"Mights" are things the heroes might learn, and they make their job easier if they do, but they don't technically need to know them to advance the narrative. These things often make the heroes feel prepared and smart to learn them, and so they're good for player engagement. "Mights" also include acquiring tools that could prove useful. (Examples: the king's evil advisor is a raksasha in disguise, the catacombs are filled with wraiths, here is a silver hammer, here is something to help protect you against the Mindwarping Gem's effects, here is a blessed crossbow bolt that can kill a raksasha easier)

When designing an investigation, think about where the heroes need to end up and work backwards, making a list of "musts" and a few "mights." You can develop more "mights" as you craft the investigation, but you need to have a clear sense of your "musts" from the outset.

Next, what to do with that list!