It’s not true that everyone in prison claims to be innocent. Most inmates I’ve known have been approximately as honest as people on the outside. Take that as you will.

What is true, in my experience, is that a guilty person and a not-guilty person sound alike when protesting their innocence. Sorting one from the other requires expertise, a lesson I learned as a young reporter when I accompanied a legendary lie-detector operator to interview a witness in a murder case.

Warren Holmes was a former Miami police detective turned independent consultant; investigative agencies around the world sought his advice. He performed polygraphs in connection with the John F. Kennedy assassination, the Watergate investigation and multiple high-profile wrongful convictions. His frequent collaborator, Gene Miller of the Miami Herald, won two Pulitzer Prizes for work done with Holmes — and now Gene was an editor sending me to collaborate.

My job was to persuade our squirrelly witness to submit to Holmes’ polygraph machine. But while Elmer Carroll was certainly dumb, he was not — despite my wheedling efforts — dumb enough. He eagerly and vehemently repeated his claim to have seen another man commit the murder for which two half-brothers were doomed to Florida’s death row. However, he could not be coaxed into taking the test.

As we left the medium-security prison where our witness was locked up for an unrelated offense, I dejectedly confessed to Holmes that I didn’t know what to believe. He answered confidently: “He’s telling the truth when he says he was there, but he’s lying when he says he wasn’t involved.” I asked how he could be so sure without the use of his machine.

“That’s just a cigar box with wires,” Holmes replied with a snort. “The value of a polygraph is the operator, not the machine.”

Well, subsequent events convinced me that Holmes was exactly right. Considerable evidence emerged to exonerate the half-brothers; meanwhile, Carroll revealed himself to be a killer. He landed on death row for a subsequent murder and was executed in 2013. The story Carroll told so fervently was almost certainly half-true, half-false. Many of the most convincing lies come smothered in a sauce of verity.

Which brings me to the present day. I wish I could put Holmes, who died in 2013, to work on the case of Donald Trump. The president protests his innocence with intensity worthy of Capt. Dreyfus himself. He’s the victim of a “hoax,” a “fraud,” a “witch hunt.” After the Justice Department announced the indictment of 13 Russians, including an oligarch close to Vladimir Putin, for meddling in Trump’s favor during the 2016 election, he nearly melted his phone as he furiously published an epic of self-exonerating tweets.

You or I might do exactly the same thing if we found ourselves targets of an unjust investigation. Few experiences are more maddening than to be wrongfully accused, whether the alleged offense is taking money from your brother’s sock drawer, flirting with a stranger at the Christmas party, cheating on a test or cooperating with foreign agents to improve your chances of winning the White House.

In other words, Trump’s furious claims of spotless innocence could be entirely consistent with the truth. But as Queen Gertrude observed to Hamlet, “the lady doth protest too much, methinks.” Surprising as this is in a veteran of show biz, Trump seems not to understand how a close-up magnifies every gesture. His jumpiness around the subject of Russia; his hand-wringing over ways to end the investigation; his rhetorical flop-sweat at the mention of the letters F, B and I — all these and more have his audience thinking: Gee, for an innocent man he sure does act guilty.

In the classic film “Rope,” Alfred Hitchcock has a pair of clever young men serve drinks to their professor over a trunk stuffed with their murder victim’s corpse. The tension comes from the professor’s slow discovery of their crime. A remake with Trump in the leading role would open with him tweeting: “There’s NOTHING in the TRUNK!”

Channeling Holmes, I would venture that Trump is telling some of the truth but not all of it. He’s a businessman whose serial bankruptcies made it hard to find U.S. banks willing to extend further credit. His search for capital took him to Russia, where it is very difficult to do deals without getting your shoes muddy. Trump may be sincere when he says his campaign did not knowingly collude with Moscow — yet may know, at the same time, that he has skeletons in Russian closets that the special counsel, left unchecked, may find.

But this is speculation, and the great polygraph operator is not available to offer his expert opinion. We must continue to be patient as Robert Mueller carefully loosens the straps to reveal whether the trunk is empty or full.

David Von Drehle writes a twice-weekly column for The Washington Post.

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