Investigation starts at 0.
"Mock him publicly" before the investigation starts → +1
Investigation (= 1)
"Thank him"
→ no penalty
Go to
Forest
"Clean up the site of the murders." →
You cleaned up the site of the murder
Go to
Silvertusk
"Talk to orc allies" → +1
Investigation (= 2),
You talked to orc allies
"Talk to Jadk" - "Can we talk about the murder investigation?" → +1
Investigation (= 3)
Go to
Central Camp
"Talk to shaman" - "Have you heard any rumors about problems with the Silvertusk Bar?"
Go to
Orc Tents → +2
Investigation (= 5)
Go to
Central Camp
"Speak to investigator" - "I've gathered all my evidence."
since
You talked to orc allies → no penalty
since
You didn't get the second witness → -1
Investigation (= 4)
since
You cleaned up the site of the murder → +1
Investigation (= 5)
To succeed, you need
Investigation >= 5
@Vinfamy made a gallery unlocker.
There is also a mod to unlock the gallery and get the bonuses ; link is in the OP.