2. “This is the single greatest witch hunt of a politician in American history!”

President Trump took to Twitter early this morning to voice his dissatisfaction over the appointment of a special counsel to investigate ties between his presidential campaign and Russian officials. At the White House later, he denied any collusion, adding: “I can only speak for myself.”

Rod Rosenstein, the deputy attorney general, above, said he knew the decision to fire James Comey was made before he wrote a justification. Mr. Comey had become unnerved by his interactions with the president since he took office.

We collected reactions from the left and right on the naming of Robert Mueller, a former F.B.I. director, as special counsel.