Every time Americans start thinking nothing more can surprise them in this presidential campaign, something detonates to prove them wrong. So it was that on Friday, 11 days from Election Day, the F.B.I. director, James Comey, sent a cryptic letter to Congress saying, “In connection with an unrelated case, the F.B.I. has learned of the existence of emails that appear pertinent” to its investigation of Hillary Clinton’s private email server.

Whose emails? About what? Do they have any connection to Mrs. Clinton herself? Mr. Comey wasn’t saying. Indeed, he appeared to have no idea. He went on to inform Congress that the F.B.I. “cannot yet assess whether or not this material may be significant,” nor could he even predict how long it would take the F.B.I. to figure that out.

In a rare moment of bipartisan agreement — if not mutual interest — the campaigns of both Mrs. Clinton and Donald Trump demanded, quite correctly, that Mr. Comey provide more information, and fast.

According to numerous reports on Friday evening, the new emails were discovered during the bureau’s investigation of Anthony Weiner, the estranged husband of Huma Abedin, an aide and confidante of Mrs. Clinton’s. Mr. Weiner is being investigated in connection with illicit text messages he sent to a 15-year-old girl in North Carolina.