As a noun, suspect refers to a person who is thought to be guilty of a crime, not to the person who commits a crime. A suspect is a certain person of uncertain guilt; a criminal (or similar title) can be an uncertain person of certain guilt. It is poor form to use suspect in place of criminal, thief, robber, murderer, etc., even if we often hear journalists and police say things like, “after robbing the store, the suspect escaped on foot,” which makes little sense in strict usage.
As an adjective, see suspect or suspicious.