The fact that Hillary Clinton exclusively used a private server in her home, rather than a secure government server, to send emails during her four years as secretary of state has raised many questions. She now says that it was a mistake but also emphasizes that she broke no law. News reports typically describe her offense as not following “policy.”

Whether or not Mrs. Clinton violated a State Department rule, her admitted destruction of more than 30,000 emails sure looks like obstruction of justice—a serious violation of the criminal law. Let’s consider some of the basic, undisputed facts, and then the law.

First, Mrs. Clinton was worried that communicating through email would leave a trail that might be subject to subpoena. “As much as I’ve been investigated and all of that,” she said in 2000, “why would I ever want to do email?” But when she became secretary of state, she didn’t have much choice. So she set up a private server in her house. That way, in the event of an investigation, she could control which emails would be turned over.

We know this is true because that is exactly what happened. When Congress subpoenaed Mrs. Clinton’s official communications, or when nongovernmental organizations filed Freedom of Information Act requests for the same, the State Department could not turn over her emails because it did not have them.

The State Department must have known that its leader was using a private account. Mrs. Clinton presumably emailed other officials within the department, and the “from” line would have shown clearly that she wasn’t sending the message from a proper government email address.

Mrs. Clinton claims that she never sent any classified or secret information on her private account, though many have noted that she conspicuously left out whether she received such material. Either way the claim is hard to swallow—one would think it would have left her out of the loop—but let’s assume she is telling the truth.

By her own admission, Mrs. Clinton destroyed more than 30,000 emails once the subpoenas started coming in. She claims that she only destroyed personal records. Team Clinton initially explained that her work emails were separated from her personal emails using keyword searches. Now, after the outcry about how much this method might have missed, Mrs. Clinton is insisting that every email was individually read before the deletion.

Still, this leaves questions unanswered. How did those reviewing the emails define “personal”? For instance, if Mrs. Clinton had emailed a foreign government about a donation to the Clinton Foundation, was that message—whatever its interest to watchdogs or voters—tossed in the trash bin?

The law says that no one has to use email, but it is a crime (18 U.S.C. section 1519) to destroy even one message to prevent it from being subpoenaed. Prosecutors charging someone with obstruction don’t even have to establish that any investigation was pending or under way when the deletion took place. As T. Markus Funk explained in a journal article for the National Association of Criminal Defense Lawyers, the prosecutor “need only prove that the defendant shredded the documents, at least in part, to make life more difficult for future investigators, if and when they eventually appear.”

Legal commentators call this “anticipatory obstruction of justice,” and the law punishes it with up to 20 years imprisonment. The burden of proof is light. The Justice Department manual advises that section 1519 makes prosecution much easier because it covers “any matters” or “’in relation to or contemplation of’ any matters.” It adds, “No corrupt persuasion is required.”

In addition, rules governing the practice of law forbid attorneys from anticipatory obstruction of justice. These ethics rules are drafted by the American Bar Association, but they are also reflected in real law. Virtually every state court adopts them, and violation can lead to disbarment. Rule 3.4 (which has been around for many years) provides that an attorney shall not unlawfully “conceal a document or other material having potential evidentiary value.” Mrs. Clinton is a lawyer governed by these rules. So are any attorneys who advised her to delete her emails.

When the Senate Watergate Committee discovered that President Richard Nixon had a very extensive inventory of White House tapes, both the committee and the special prosecutor subpoenaed them. At the time, some wondered why Mr. Nixon, a lawyer before entering politics, didn’t simply destroy the tapes. The answer was that doing so could have led Mr. Nixon to an indictment for obstruction as well as disbarment.

At the time, I was assistant majority counsel to the Senate Watergate Committee, so I remember that period well. Mrs. Clinton should remember it too: She worked on the House Impeachment Committee, which warned Nixon not to destroy the tapes.

Here’s another reason Mrs. Clinton should know about obstruction: Congress enacted section 1519, making the crime easier to prove, in 2002, as part of the Sarbanes-Oxley Act. As senator, she voted for the law.

Mr. Rotunda is a professor at Chapman University’s Fowler School of Law and the co-author, with John Nowak, of “Treatise on Constitutional Law” (Thomson Reuters, fifth edition, 2013).

Source: Ronald D. Rotunda, www.wsj.com