For the House of Representative committees want to issue subpoenas in an impeachment, they need a full House vote authorizing the impeachment investigations. So far, no subpoenas have been issued because there has been no House vote. From sundance at theconservativetreehouse.com:
The House democrats will keep doing this until someone in the media begins to hit them with hard questions that expose the nonsense.
Today Chairman Adam Schiff (House Permanent Select Committee on Intelligence); Chairman Elijah E. Cummings (Committee on White House Oversight; and Chairman Eliot L. Engel (House Committee on Foreign Affairs) continue sending carefully worded letters under the guise of ‘subpoenas’. [Main Link Here]
Again, just like all prior examples, this is not a “subpoena”, it is a letter calling itself a “subpoena” and carries NO legal penalty for non-compliance. A legislative “letter” needs to carry judicial enforcement authority –A PENALTY– in order to be a “subpoena”.