"他们是谁？"

Translation: Who are they?

[borisdekloris]

Why is there no ma at the end?

[Swansae]

吗 is only used for yes/no questions

[varigby]

谁 already turns it into a question.

[Thomas292098]

The word 谁 is already a question word meaning "who?", so it would redundant and unnecessary to add 吗 to the end.

*Usually you need 吗 to change a sentence into a question that does not already contain a question word....
Eg:
(你吃了, you've eaten)
(你吃了吗，have you eaten?)
(你吃了什么，what did you eat?)

[phszombie]

吗 is used if you are asking a question which requires a yes/no answer. You cannot answer "Who are they?" with a "yes" or "no."

[Andrew590503]

Ma is just a particle, or a classifier, so we might not use it when talking about talking about people first person. I'm not a pro, but I predict it.

[akavel]

I think it's beacuase "who" already means a question

[zest16]

Is it pronounced "shuí" or "shéi"?

[michisjourdi]

The second one

[MaurieMill]

Because shei (who) functions as a question particle because it is obviously a question. Just like if anyone says who, what, where, when, or why in English. It is assumed to be a question.

[LilyInTheRough]

Related: can I use the particle "ma" in the end? Would it emphasise it as a question or so?

[weirdnerdykid123]

can i...EVRY BODY DO THE FLOP!!!!!!!!!!!!!!