Complex sentence: Morrison's skull and some other bones were erroneously mislabeled as Valenzuela's when it was in storage, which investigator Nikki Costa attributed as one of the reasons why the remains were unidentified for so long.
Split into simpler sentences: Morrison's skull and some other bones were mislabeled as Valenzuela's while they were in storage. Investigator Nikki Costa said this was one of the reasons why the remains were unidentified for so long.