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を書いたのは

… was the one who wrote

→ を
phrase Uncommon #17,946
… was the one who wrote Used to identify or ask who wrote something. The thing written appears before this phrase.
JA: 手紙を書いたのは姉です。
EN: My older sister was the one who wrote the letter.

それを書いたのは私じゃないです。

I didn't write that.

これを書いたのは白人だな白人だろう？

A white person wrote this, huh? They must be white, right?

— マルコムＸ (1992)

あの報告書を書いたのはあなたですか。

Were you the one who wrote that report?

メモを書いたのはメアリーだと、トムは知っていた。

Tom knew that Mary had written the note.

壁にあの落書きを書いたのは誰だと思う？

Who do you think wrote that graffiti on the wall?

そして君が足りないと言う脚本を書いたのは？

And who wrote the script you say is lacking?

— Pompo the Cinephile (2021)

これを書いたのはあなただったの？

Were you the one who wrote this?