The case is still open
The earlier reports were erroneous, giving the impression that Natalie Wood's death case has been closed, but while it is classified as a cold case (which proves that the authorities don't believe her death was accidental), the case is still open and the investigation is ongoing:
http://www.accesshollywood.com/natalie-wood-death-investigation-a-cold -case_article_58612
It seems that some are just too eager to jump to conclusions regarding the status of the case.