Near the end of season 2, it is finally revealed that the body unearthed from Alex's house is that of Sara's mentally disturbed father, Abel. Initially seen in a mental institution where Sara tracks him down, Abel soon convinces his paranoid daughter to help him bribe his way out of captivity.

Who did Alex dig up?

The season one finale implies that the body is Sara herself since it's her death that has been the driving force behind the whole show and the final scene sees Sara being attacked before César shoots. However, it's finally revealed the body is not Sara, but rather her birth-father Abel Martinez Osorio.

Who was buried in Alex's backyard?

While it doesn't resolve the title question, Who Killed Sara?'s season 2 finale does answer several questions that were posed from the previous season, such as the identity of the body buried in Alex's backyard, which turned out to be Sara's father, who César shot and killed to save Sara's life.