のは誰だ

who ...?

→ の
phrase Common #4,106
who ...? Used to ask who performed an action; の turns the preceding action into “the one who did it,” and 誰だ asks who that person is. The plain ending 誰だ can sound direct or emphatic.
JA: この絵を描いたのは誰だ?
EN: Who painted this picture?

窓を割ったのは誰だ。

Who broke the window?

さてこれをしたのは誰だ?

Now, who did this?

— 罪人たち (2025)

シヴァガミという女を支えるのは誰だ!?

Who is backing that woman, Sivagami?!

— バーフバリ 伝説誕生 (2015)

そこにいるのは誰だ。

Who’s there?

俺と一緒に死ぬのは誰だ?

Who will die with me?

— バーフバリ 伝説誕生 (2015)

共に来るのは誰だ!

Who’s coming with me?

— バーフバリ 伝説誕生 (2015)

顕微鏡を発明したのは誰だか知ってる?

Do you know who invented the microscope?

空を見上げているのは誰だね。

Who is looking up at the sky?

恥ずかしいなら帰れ来いって頼んだのは誰だ?

If you're embarrassed, go home. Who asked you to come?

— オープン・ユア・アイズ (1997)

石を投げて窓を割ったのは誰だ。

Who broke the window by throwing a stone?

誰かが泣いている泣いているのは誰だ?

Someone is crying. Who is it that's crying?

— 羅生門 (1950)

マッダレーナと一緒にここに来たのは誰だ?

Who came here with Maddalena?

— 甘い生活 (1960)