Spiral - Suiri no Kizuna - Vol. 1 Ch. 1 - The Invisible Murderer in the Balcony (First Half)

[MangaDex]

 

[BakedBanana]

I can already tell I'm not going to like this Wataya Suemaru character on just page 12 alone.

Here's my guess:

Spoiler: Guess (not spoilers)

The victim leaned over the fence when she was called by the culprit and while she was waving downwards the fence broke, the culprit probably fiddled with the fence since you can see bandages on her hands on page 56. The culprit probably pocketed the letter she sent out (Page 53) after she fell.