diào / chá / rén / yuán / míng / chá / àn / fǎng / zhōng / yú / zhǎo / dào / le / wèn / tí / de / gēn / yuán

调查人员明察暗访，终于找到了问题的根源。

The investigators conducted both open and covert inquiries and finally found the root of the problem.