I'm lost on how it was done, after Apollo figures out that L'Belle was disguised as Tenma. How did the door get locked from the outside if the key was found inside and the air vent wasn't used to put the key back in the room?

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*ahem*

There are two keys. The master key and the sunflower key. Jinxie had the master key at all times (she uses it to unlock the door) and the sun key is ultimately irrelevant to the murder. It's a red herring

L'Belle drugged Tenma and the Alderman, went in the room, locked the door, and murdered the Alderman. He then unlocked the forbidden chamber and put Tenma inside. He closes the forbidden chamber again. L'Belle now dons the Nine Tails mask and screams. Jinxie comes in, thinks L'Belle is her father, and then leaves. The door is now OPEN. L'Belle walks over to the Forbidden Chamber, opens the door, and Filch escapes (he crawled through the vent). L'Belle drags Tenma onto the chair and leaves, throwing the mask out the window

Jeez this case is a mess. Thanks. I completely forgot about the master key.

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We need more mechanical breakdowns like that. Some cases can just be difficult to wrap your head around from memory (lookin at you 3-5)

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And there we have it...

Another key is unaccounted for in your reasoning. This one being the fox chamber key.

It can be said that Lbelle after drugging the alderman and the mayor was in possession of it. And who opened the forbidden chamber to Filch after the "confession". Well Lbelle naturally. And then we come to the logic breaking point.

If Lbelle was in possession of the fox chamber key, how was it found in the mayor stomach?

Now I can guess the beginning of an answer, this one being that Lbelle framed Mayor Tenma with it. And here we have another mystery. The mayor being unconscious.

Let's recap a litle here. Mayor tenma was drugged and clocked however he managed to be in a semi conscious state probably because of the ruckus made by the door opening but he didn't stay awake very long. In fact one could assume he fainted right away.

My point is as follow, after carrying the mayor to the couch, throwing the mask by the window Lbelle would have given him the fox chamber key and left the room, but when would the mayor have eaten it, he was inconscious all this time.

I think the truth wasn't reached in this trial, while Lbelle act was suspicious he probably lost all will to fight once learning he was chasing something that didn't exist. In other words... Lbelle wasn't the one that opened the Fox Chamber room.

LBelle opened the fox chamber, put it on the mayor, the mayor woke up, and then swallowed it. He said he swallowed it once he was lucid. No contradiction here

When was it then?

He wasn't lucid when appolo found him, and the police was already on their way.

Tenma was drifting in and out of consciousness. He remembered the Amazing Nine-Tails (actually L'Belle) opening the Forbidden Chamber from the Fox Chamber, and then after he left Tenma swallowed the key. Apollo found Tenma in a dazed state and then presumably lost consciousness, as the prosecution claimed he was found unconscious.

And this is the SECOND case.

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Thing is that I actually figured out the whole trapped island thing quite a bit before the complete explanation was given. On this case however, the keys, and the potential "where could L'belle have hidden" had me SO confused.