investigate|investigating

约翰去了他的办公室并开始调查绑架案。

John went to his office and started ___investigating the kidnapping.

我们当然在调查这样一个错误是如何发生的。

We are of course ___investigating how an error like this could have occurred.

警方正在调查涉及高级管理人员的腐败指控。

Police are ___investigating allegations of corruption involving senior executives.