So, here's the plan how we can prove that Umbridge was the mastermind behind this:

So @Fictiongirl250 aka Wildpaws: 

I expect a daily update about your mission (see comments from the last chapter) on my message board.

@Sadiekane173 aka Bhavika:

I want you to look after possible soon to be victims!!!

I just want you to tell me who you suspect is the victim on my message board. And then, your job is to keep an eye on them, making sure that they stay safe.

@Winter_Solstice24 aka Az:

Your job is to keep your eyes open for suspicious acts and possible murder attempts.

If you notice anything suspicious, please report it immediately to Wildpaws, Bhavika, and me on my message board.

Also, your job is to try to find more information and proof!!!

Ask teachers indirectly about the victim, murder case, and become a detective!!!

Read the daily prophet or do whatever you have to do!!!

You need to help me with this case!!!

@BlossomNyx aka Nyx:

Your job is one of the most important:

You need to help me break into the ministry of magic and help me find the identity of the victims!!!

@Skulduggery_JY:

I want you to try to become Umbridge's favorite!!!

Maybe she will trust you with a few pieces of information then.

If you fail, we all will fail!!!

@wolfstar_is_canon aka Liv:

Help Az (@Winter_Solstice24) and become a detective, too!!!

I have a feeling that you and Az would make a great team!!!

Also, your job is to send letters to people outside Hogwarts and gather trustworthy and powerful allies!!!

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To all the others who weren't mentioned:

Don't worry, you are all still needed!!!

Help the people mentioned above as much as you can!!!

Inform them and me, if you see suspicious acts, etc.!!!

We can do all of this 10 times faster if we all stick together!!!

And we need a bit of money to do all of the planned, so gather money!!!

And recruit people!!!

Give this Group Chat a HUGE shout-out.

More people need to be aware of what is going on at Hogwarts!!!

Well, this is it for now.

If there is more, I will update it!!!

I am expecting you all to help me with this case and take your roles in this case seriously!!!

See you all (hopefully) in that next part!!!