Passive voice structure refers that the subject of the event is unknown or can be deleted. The subject in a passive voice sentence is called patient or receiver and the main subject in the same sentence either is known agent which is associated with the preposition 'by'. In contrast there are many sentences do not have an agent which may not be known for the writer. In this text, the agent is not specified in any of the 4 sentences .the active voice was not available to the writer because she/he was unable or did not want to write the name of the agent. For example 'the Brutus case could be developed' in line 10 and 'it should be shut down' in line 11 and 'the case was closed, in line 13 and 'it was decided to keep' in line 18.In the first and second sentences, the writer uses the passive voice with the future tens .But in the first one by the modal verb 'can' which refers to possibility of developing in real means of deceiving enemy. In the second one by the modal verb 'should' which refers to give suggestions to the spy .in addition , the subject in the two sentences( it and the Brutus case) are called patient. In the third sentence, the writer uses the passive voice with past simple in the second clause and he/she possibly does not know who closed the case. In the fourth sentence, the writer uses the passive voice with the future form in the second clause and he / she does not know who decided for the case was still running.