Summary: Oddly enough, it wasn't so much of what Mr. Comey said in the House intelligence Q&A, but it was the volumes of what he said he wouldn't say that was so damning. By announcing the investigation into the Trump campaign without naming or excluding anybody, including Donald Trump himself, the FBI Director essentially placed anyone even remotely close to the president on notice, that they could be a suspect. This is huge news by any measure. Stress and blood pressures levels at the White House had to be spiking throughout the entire session.
