知道、了解有关情况的人someone who is in the know

• 据知情人透露，那份租赁合同，实际上是在地区行署的一间办公室里签的。

  According to a knowledgeable source, that rental contract was actually signed in an office of the regional government.

• 举报时最好能具体说明其他知情人等情况。

  When reporting, it is best if one can be specific about things such as other people who are aware of the situation.