The correct answer to this open question is the following.
You did not include any reference for this question. We do not what kind of case you are talking about. The Senate has held many cases. What is the one you are referring to?
However, trying to offer some help, we can comment on the following general terms.
When the United States Senate conducts investigations, they form special committees with specialized legislators that try to focus on a special issue. The members of the committee have the power to invite to declare the people involved in the issue when it is needed for the investigation purposes. The committe analyses, discuss, and comes up with conclusions that are established in a final report.
The founding fathers and the framers of the Constitution did not refer particularly to the power of the US Congress to initiate investigations. However, as the Constitution says that the US Congress has all the powers, then it is understandable that Congress can develop political and economic investigations.