For those involved with shady business, or perhaps simply underhand politics in Washington – such as Michael Flynn – it is commonplace for witnesses to ask for #immunity in exchange for their testimony, which then ensures that what they say can't be used against them.
What federal law says about congressional immunity
Under The United States Federal Law, those in Congress researching and investigating serious breaches of the law and political systems, can give witnesses immunity for their evidence, but it's only usual for official lawmakers to do this after conferring with legal prosecutors.
Usually congress would avoid taking any action that could interfere with a national or #federal investigation. And interestingly, federal law states that the Justice Department is allowed to delay a case of congressional immunity but not void it altogether.
With the case of #Michael Flynn and the Russian probe being undertaken by House and Senate investigators, it's unclear what path the Justice Department will take.