Is it permissible to pray behind an imam who performed the funeral prayer over an atheist, while knowing of his atheism?

If the deceased was truly an atheist, then it is not permissible for anyone who knew his condition to pray over him, due to the Almighty's saying: (And do not pray over any of them who dies - ever). If the Imam was ignorant of the deceased's condition or doubtful, he has acted according to the default rule. However, if he knew of his atheism, then he has erred. But one cannot judge the Imam before knowing the reason for his action.