Investigations, criminal and otherwise, need cooperative witnesses to supply evidence. In some cases, that assistance is forthcoming -- people report crimes, testify against defendants, or hand over DNA, documents, and other physical evidence. In other cases, witnesses are a little more reluctant.
Take the case of retired Lt. General Michael Flynn, President Trump's former national security adviser and center of two investigations (one congressional and one criminal) into the Trump campaign's contact with Russia before and after the 2016 presidential election. Flynn is not eager to testify in front of Congress, so the Senate Intelligence Committee issued a subpoena for his testimony and related documents. Flynn, through his lawyers, politely declined the request. So what happens now?
A subpoena is a formal document that orders an individual to appear before and provide testimony to an authorized investigative body. Subpoenas can also demand the production of related documents and are usually reserved for recalcitrant witnesses who indicate they will not appear and provide for civil or criminal penalties if they are not obeyed.
The subpoena in this case demanded Flynn appear before the Senate Intelligence Committee and provide a list of any contacts he had with Russian officials between June 16, 2015 and January 20, 2017.
In response to the subpoena, Flynn pleaded the Fifth. Invoking the Fifth Amendment's protection against self-incrimination means that the government may not compel any person to give "testimonial evidence" that would likely incriminate him in a subsequent criminal case. The right against self-incrimination is not absolute: it only applies to testimonial evidence (as opposed to evidence used for the purposes of identification like fingerprints, DNA, etc.) and only applies to individuals -- corporations may not invoke the Fifth when served with a subpoena.
Which is why Intel Committee leaders are serving Flynn's Virginia-based businesses with the two new subpoenas, requesting any documents related to his contacts with Russian officials. Flynn will likely still resist those subpoenas as well, arguing that any such documents are testimonial in nature and even the act of production could be testimonial.
So what recourse do investigative bodies have if someone invokes the Fifth? The person can be charged with contempt of court and face financial sanctions or jail time. Most charges of criminal contempt involve a specified jail sentence, and in Flynn's case he could be looking at up to 12 months in jail if he is found in contempt of Congress. But that might not happen any time soon -- the Senate Intelligence Committee would need to vote to recommend holding Flynn in contempt, and then the full Senate would have to vote in favor of contempt before court proceedings could be started.
Before you decide to plead the Fifth or otherwise refuse to comply with a subpoena, talk to an attorney first.