But with polls showing Mrs. Clinton holding a comfortable lead, Mr. Comey ended up plunging the F.B.I. into the molten center of a bitter election. Fearing the backlash that would come if it were revealed after the election that the F.B.I. had been investigating the next president and had kept it a secret, Mr. Comey sent a letter informing Congress that the case was reopened.

What he did not say was that the F.B.I. was also investigating the campaign of Donald J. Trump.