If your boss thinks you may have dipped into petty cash to pay your gambling debts, you may be suspect (or “a suspect”). But if you think somebody else did it, you are suspicious of them. Confusingly, if the police suspect you of a crime, you can be described as a “suspicious person” and if you constantly suspect others of crimes, you can also be called “suspicious.”
But “suspect” is not so flexible. A suspect is a person somebody is suspicious of, never the person who is doing the suspecting. It never makes sense to say “I am suspect that. . . .”
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