Investigations conducted by DOJ are completely separate from Congressional oversight. Congressional oversight has nothing to do with investigations and prosecutions in the executive branch and Congress doesn't "participate" in investigations by the Justice Department. The Justice Department's investigatory and prosecutorial powers are conducted within its own jurisdiction by its own investigators and prosecutors. Congress has nothing to do with it. Congress has oversight power, but that power is totally within its own jurisdiction is neither a prosecutorial power nor does it have any connection to the executive branch's power to investigate and prosecute.