The agent
The agent, that is, the doer of the action in the active voice, is normally added at the very end of the passive clause or sentence. It is important that you remember that when the agent is added, it is preceded by the preposition by.
Have a look at the following examples, paying special attention to the sentences in which the agent is added.
Paco was given a present by Sonia (Active: Sonia gave Paco a present).
Paco has already been told the truth (Active: Someone has already told Paco the truth).
The decision had not been made yet by the Committee (Active: The Committee had not made the decision yet).
It is said to be true (Active: People say it is true).
One of the reasons why we use the passive voice in English is that we are not interested in the doer of the action. However, we may mention it if it is known. If the doer of the action is not specifically known (someone, somebody, people, etc.) the agent is left out in the passive sentences as in the two examples above.