Due to public pressure, the government decided to re-open the old investigation.

re-open 是由前缀 re-（表示“再次”）和动词 open（打开、开始）组成的复合动词，意思是“重新开放”或“重新开始”。在这里指“重新调查”，即对已经结束的调查再次展开。