Next in line to the oval office

Never before in the history of the U.S. have the outgoing and incoming presidents and vice presidents been killed on Inauguration Day. By law, the Speaker of the House should be sworn in as president, but he is not able to serve. Next in line would be the President of the Senate Pro Tempore, but he is not sworn in. A new Speaker is named, and she is given the Oath of Office. The Senator vows revenge. Meanwhile, the search goes on to determine who caused the three explosions on the west portico of the Capitol, why, and how. The plot twists and turns, with an O. Henry-type ending, and includes a "Deep Throat II," whose identity will fool almost all readers.