Verb する in this sentence


A building collapsed, but everyone had already evacuated it. Then someone says
the following sentence: 全員が避難した後でしたので 幸い 死者はなし

I did not understand the meaning of the verb する after 後で. Could someone
explain to me?

お返事ありがとうございます！




There is no する after 後. It's でした, the polite form of だった.

全員が避難した後でしたので  
= 全員が避難した後だったので  
= because it was after everyone evacuated

