Did Futaba's mother work for the Kirijo Group? I was hesitant at first, because men in black suits are sometimes just men in black suits.
But then I got to thinking that its pretty curious that a suicide would be covered up by men in black suits with a forged suicide note. Which made me think it was more of an occupational death. And if she was researching the cognitive psience side of Shadows, before the original Gekkoukan exploded, then that would give the Kirijo Group (or even the Nanjo Group if you wanted to bring this back into P1/P2 continuity) a good reason to conceal the reason she died or to stop her (or her research) from going public.
Or if we want to get really sinister, Futaba was a candidate to have potentially been picked by the Kirijo Group for Dark Hour research into Tartarus like Strega was, but she was too young to have begun her medication.