Hillary Clinton waves to supporters after speaking at the National Education Association’s representative assembly in Washington on July 5. (Kevin Lamarque/Reuters)

FIRST, A shout-out to the investigator: In examining the use of a private email server for official business by then-Secretary of State Hillary Clinton, FBI Director James B. Comey appears to have navigated treacherous waters with the commitment to principle and rule of law that any citizen could wish for. The FBI investigated thoroughly. It came to its conclusion in time for voters to factor it into their deliberations. Mr. Comey leveled with the American people, reporting to them even before reporting to his own bosses.

And the conclusion he came to strikes us as being as sound as the process: Ms. Clinton was “extremely careless,” the FBI director said, but did not engage in the kind of intentional or willful mishandling of classified material that would warrant prosecution.

The director left no doubt of his belief that Ms. Clinton’s use of a private email server was entirely inappropriate for a Cabinet member with access to the nation’s highest-level secrets. Mr. Comey said “any reasonable person in Secretary Clinton’s position . . . should have known that an unclassified system was no place for that conversation.” The Clinton system had virtually no archiving; when servers were swapped out in 2013, “millions of email fragments” wound up being inadvertently tossed into the “slack space” of a decommissioned server. Much will be made by Ms. Clinton’s critics of the finding that eight chains of “top secret” emails were found, as well as several dozen chains of emails at lesser levels of classification; and that work emails were missed by her lawyers in their sorting from the personal; and that thousands of other emails were dislodged by investigators from other people’s accounts.

No doubt an FBI investigation of equal intensity would find carelessness on the part of many other high-ranking officials. But that does not excuse Ms. Clinton’s blithe disregard for procedure. The FBI found no “direct evidence” that Ms. Clinton’s email was hacked by foreign adversaries, Mr. Comey said. But it is worrisome that the FBI believes “hostile actors gained access” to private, commercial email accounts of her interlocutors, and that it is “possible” her emails also fell into hostile hands. China’s and Russia’s known, aggressive cybersnooping may have reaped a rich harvest.

Carelessness, however, is not a crime. Mr. Comey said there is no precedent, in his view, that would justify prosecution in this case. Once questions were raised, Ms. Clinton turned over some 30,000 emails — in the aggregate, probably more email than any of her predecessors preserved.

FBI Director James Comey said on July 5 that Hillary Clinton should not be charged for her use of a private email server during her time as secretary of state. Here's what he said, in three minutes. (Peter Stevenson/The Washington Post)

From what’s come to light, it seems clear Ms. Clinton and her aides used the private email server to preserve control over her messages, neglecting their responsibility as public servants to follow procedures for protecting classified information. Rather than toss off this experience with a back of the hand, Ms. Clinton needs to learn from it and find a way to show voters that she has better judgment than the combination of high-handedness and defensiveness she has displayed here.