No, you can't say that. You can say 'Who is at fault?', or more commonly, 'Whose fault is it?'

yes i got it :)

As a student of Arabic, I would say that Arabic and English prepositions rarely match, so it's better to not try and translate the prepositions directly. I have to learn آمن بشيء and not آمن في شيء and so on.

aha .Thanks a lot :)

Who is "at" fault is more common in legal proceedings.