Here, the verb catch is a transitive verb, but the object/agent is missing. The action of catching a thief devolves onto the police or the people. So, we need to create the agent as per the context. The thief has been caught by the people/ police/them. Consequently, the active form of the sentence will be: The police has caught the thief. The people have caught the thief. They have caught the thief.
