Where does it say that? He applauds his team for the work done. He signed his name on first page. No where on first page does it say they are still continuing.
Where are you reading they are still investigating?
In the article, FBI source said the emails on laptop were 10 years old and any emails there were just duplicates of what they have reviewed.
Read his letter. It was very narrowly focused: The private server and only e-mails to or from Clinton during her stint as SOS. Everything else is still open.