Who Killed My Sister?

When I came home, I found my sister dead. She has left me a coded message.

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To: My Brother

A・ C・ F・ ?・O

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Even in shock, I knew that by solving the riddles I could seek the truth no matter how horrid it may be....

The further I get in solving riddles, the closer I get to the suspect.

But the suspect has a contradicting testimony.

-Will you find the killer?-