Are we positive that there isn't some rule of law that prevents/should have prevented the FBI from going into Jefferson's office? Maybe the raid had to be requested by Congress. Don't they try their own? (not that justice is actually done...)
Jefferson's office was contacted by the FBI before they went to a judge, and his office denied the FBI access. And now we know why.
The judicial branch granted the FBI the authority to search the Congressman's office after the FBI presented enough of a case to support the need to search for evidence of a felony.
What would possibly protect a Congressman, who at last count is an American citizen and subject to American law, from being investigated?
Are you asking if Congress has the power to investigate a felony committed by a fellow member?
Congress seems to think so.