In the end, by-the-book Robert Mueller departed wildly from the book.

He invented an extraconstitutional legal standard for his obstruction investigation and acted, at best, in violation of the spirit of the special-counsel regulations.

His departing act was a public statement meant to influence the public debate in a manner inappropriate for a prosecutor, in part because the long public report he wrote that was also inappropriate for a prosecutor lacked clarity.

A hallmark of the Trump era is that the norm-defying president goads everyone appalled by him to violate norms. The former Marine and G-man Robert Mueller would seem least likely to fall prey to this dynamic, but here we are.

First and foremost, Mueller ditched the presumption of innocence. In the normal course of things, all of us are considered innocent unless a jury finds us guilty beyond a reasonable doubt. Mueller switched this around. Rather than finding conclusive evidence of Trump’s guilt, he had to find conclusive evidence of his innocence. Since he didn’t find this exculpatory evidence, Mueller deemed Trump “not exonerated.”

In surely one of the more gobsmacking utterances ever made by someone from a Justice Department podium, he said that “if we had confidence that the president clearly did not commit a crime, we would have said so.”

If this standard had been applied to any person other than Donald Trump, it would have been widely denounced, and the American Civil Liberties Union would be crusading against it.

Naturally, Trump’s critics immediately concluded from Mueller’s statement that if Trump hadn’t been found innocent, he must be guilty. The snarky headline on a New York magazine piece put it aptly enough: “Mueller: Trump is Not Not a Criminal.”

Well, you might say, of course Robert Mueller departed from standard operating procedure — he’s a special counsel operating in novel circumstances that require novel approaches.

But this is not his writ. A special counsel, under the regulations, has the “investigative and prosecutorial functions of any United States Attorney.” He is supposed to “comply with the rules, regulations, procedures, practices and policies of the Department of Justice.”

One of those rules, set out by the department’s Office of Legal Counsel, famously says that a sitting president can’t be charged with a crime. Mueller explained in his public statement that this ruling led his office to conclude that it couldn’t “reach a determination one way or the other about whether the president committed a crime.”

So, Mueller, by his own account, conducted a two-year investigation knowing from the beginning that he wouldn’t make the either/or decision that prosecutors exist to make.

Mueller offered several reasons for conducting an obstruction investigation anyway. One was to preserve evidence. But for what? The idea that a post-presidency Trump will get prosecuted as a private citizen for firing FBI Director James Comey is manifestly absurd. Another was that the president can be accused of wrongdoing in “a process other than the criminal justice system,” i.e., impeachment by Congress.

Here, Mueller is basically describing Volume 2 as an impeachment referral. But this isn’t a purpose contemplated by the special-counsel regulations, which say that the special counsel “shall provide the Attorney General with a confidential report explaining the prosecution or declination decisions.”

They don’t say a special counsel shall fail to reach a prosecution or declination decision, then write a long report for congressional consumption anyway, and additionally go out and make a public statement to catalyze congressional action.

It should be up to Congress, not an inferior executive branch, to launch such an inquiry or not. Even more bizarrely, since Mueller considered Trump’s public statements as potential obstruction, any objections by the president to this quasi-impeachment inquiry were more evidence of alleged wrongdoing.

An institutionalist who lost his way, Mueller will be lionized for the duration since he’s been so useful to Trump’s opposition, but his performance won’t age well — and shouldn’t.

Rich Lowry is a syndicated columnist. He can be reached via e-mail at: comments.lowry@nationalreview.com.