We say "suspected" (as an adjective) to refer to a person or thing that we think may have done whatever, some specific act, or be whatever, some specific object.
We say "suspicious" to refer to a person or thing that is of a type or character that leads us to think that they may do bad things.
For example, suppose a crime has been committed, say a murder. You might say, "Fred Jones is the suspected killer." That is, the authorities think that Fred Jones may have committed the crime.
But if you say, "Fred Jones is a suspicious character", you mean that he seems like the sort of person who might commit a crime, but you are not accusing him of any particular crime. Maybe he has committed crimes in the past, maybe you just think he "looks like a criminal", whatever.
So someone might well say, "Fred Jones is suspected because he is suspicious." That is, the fact that he seems like the sort of person who would commit a crime leads us to think that he might have committed this particular crime.
Note that "suspected" can also be a verb. "The police suspected Fred Jones."
"Suspicious" can also mean that a person is distrustful. Like, "Ever since her purse was stolen from her desk, Alice has become suspicious of all her co-workers." That is, "suspicious" has two almost opposite meanings: it can mean that a person is the sort of person we think would commit a crime, or it can mean that a person thinks that others might commit a crime. If you just said, "Bob is a suspicious person" with no context, it wouldn't be clear if you mean that he acts like a criminal or that he thinks that others are criminals.
I've been saying "crime" here, but "suspected" and "suspicious" don't necessarily refer to literal crimes, just any undesirable activity. You could say, "What made Bob gain so much weight? Those candy bars he eats are suspected."
Sometimes people even use these words for positive things. Like, "Sally was the suspected source of the anonymous birthday cards."