What part of speech is were imprisoned in the sentence Daedalus and his son Icarus were imprisoned in a great maze?

In the sentence "Daedalus and his son Icarus were imprisoned in a great maze," the phrase "were imprisoned" functions as a verb. Specifically, "were" is a linking verb (the past tense of "to be"), and "imprisoned" is the past participle of the verb "imprison," together forming the passive voice verb phrase that indicates the state of being imprisoned.