YOU SAID:

Who could have done this?

INTO JAPANESE

人はこれを行っている可能性が?

BACK INTO ENGLISH

Could people have done this?

INTO JAPANESE

人これを行っているか。

BACK INTO ENGLISH

Who would have done this.

INTO JAPANESE

人はこれをやっているでしょう。

BACK INTO ENGLISH

People who are doing this.

INTO JAPANESE

これをやっている人。

BACK INTO ENGLISH

People who are doing this.