Tenemos una oración con el verbo en voz pasiva y un complemento del verbo introducido por la preposición "por"¿es obligatorio un complemento agente? Porque

no

Explanation:

The agent complement expresses in passive sentences who performs the verbal action. It designates the agent, that is, the person or thing that performs the action of the verb. It corresponds in the active sentence with the grammatical subject.

Passive sentences are written many times because the person who performed the action is not known or is not meant, so it is not mandatory to specify the agent complement in the passive sentences.

Mi hermano fue arrestado por exceso de velocidad.

This example does not say who my brother was arrested for, but rather the reasons for his arrest.

My brother was arrested for speeding.

This statement with agent complement would be:

Mi hermano fue arrestado por la policía por exceso de velocidad.

The police arrested my brother for speeding.