Although the interrogative pronouns normally takes a singular verb, if the speaker/writer thinks that there are several thieves, then the plural verb is fine.
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Mister MicawberAlthough the interrogative pronouns normally takes a singular verb, if the speaker/writer thinks that there are several thieves, then the plural verb is fine.Even so, "Who are behind the thefts," on its own, sounds extremely unusual to my native ears.
Mister MicawberAlthough the interrogative pronouns normally takes a singular verb, if the speaker/writer thinks that there are several thieves, then the plural verb is fine.I've never heard that explanation. If the person thinks that several people are behind a robbery, surely he will see them as a group/gang.