Carl Bernstein, left, Ben Bradlee and Bob Woodward gather at The Washington Post as the identity of their secret Watergate source, Deep Throat was revealed in 2005. They kept his identity secret for more than 30 years. (Katherine Frey/For The Washington Post)

Almost a year later, we still don’t know.

Outside of a tiny circle of insiders, no one knows who wrote the instantly viral op-ed column about President Trump that appeared in the New York Times last Sept. 5. Despite an informal White House investigation, plenty of outside sleuthing and a whole Internet’s worth of guessing, his or her identity remains unknown.

The column — “I Am Part of the Resistance Inside the Trump Administration” — set social media aflame and cable chat shows ablabber with speculation about who the “senior official” behind it could be.

Kellyanne Conway? Mike Pompeo? Nikki Haley? Mike Pence? In the year since publication, dozens of names have been floated. All have denied it, sometimes ostentatiously. No one has stepped forward or conclusively been shown to be the author.

History suggests this cannot last. Others who started out as anonymous in high-profile cases have eventually been revealed.

The sensation surrounding the Times op-ed writer began almost instantly upon publication of the 900-word column. The article described in general terms efforts by White House staffers, including the author, to thwart Trump’s “amorality” and “impulsiveness,” which had resulted, according to the writer, “in half-baked, ill-informed and occasionally reckless decisions that have to be walked back.”

The most outraged reaction came, perhaps predictably, from Trump himself. “If the GUTLESS anonymous person does indeed exist, the Times must, for National Security purposes, turn him/her over to government at once!” he demanded in a tweet a few hours after the column was published. Trump never explained what aspect of national security was imperiled by the unflattering column. Nevertheless, he thundered: “TREASON?”

Prompted by the president’s pique, the White House conducted a brief hunt for the writer. Aides reportedly compiled a list of suspects. There was talk of administering lie-detector tests or seeking sworn statements, though nothing seems to have come of it.

Then-White House press secretary Sarah Sanders tried a novel approach: She posted the Times’s main phone number on Twitter, and she urged people who “want to know” the identity of “this gutless loser” to call and ask the paper’s editors for his or her name. The phone campaign did not work, either; some people called the Times to praise it for running the column.

The Times’s editors said they took the rare step of shielding the writer’s identity to protect him or her from “reprisals” — which, judging by Trump’s explosion, seemed like a possibility. They said they verified the person’s identity through direct contact with the author and the “testimony” of an equally anonymous “trusted intermediary” who brokered the article to the newspaper.

In the year since, the op-ed itself has largely been forgotten, new sensations and scandals burying it daily like the sediment of successive civilizations. But questions remain: Is the author still working in the administration? What policies or initiatives did she or he actually thwart, if any?

And, of course, the big one: Whodunnit?

The fact that Anonymous remains anonymous seems like more than just a loose end. In an age of oversharing and TMI, it is tantamount to a small miracle of restraint and discretion.

To date, only five people know — or are known to know — the identity of the author. They are Times publisher A.G. Sulzberger; editorial page editor James Bennet; op-ed editor James Dao; the “intermediary;” and the author him/herself. (The list of those who presumably don’t know includes the journalists in the Times’s newsroom, among whom are some of the finest investigative reporters in the world.)

It is possible that someone inside the administration knows, too, thanks to a clue the author dropped in the op-ed. The writer quoted “a top official” who complained to him or her about a meeting in the Oval Office in which Trump changed his mind about “a major policy decision.” The author quoted this official as saying, “There is literally no telling whether he might change his mind from one minute to the next.”

Bennet and Dao declined requests for an interview for this story. In a statement, however, Bennet wrote: “As we said a year ago, this op-ed offered a significant first-person perspective that had not yet been presented to Times readers, describing the efforts made by some inside to carry out policies they believed in while containing what they saw as the president’s troubling impulses. The substance of that piece has been borne out by reporting at The Times and elsewhere over the past year.”

Journalists protect sources regularly, but Anonymous isn’t just any old source. In the few instances in which there was public curiosity about an anonymous source or writer, the person’s identity eventually became known.

Watergate’s Deep Throat, for example, outed himself after more than 30 years. The anonymous terrorist known as the Unabomber provided one clue too many and was unmasked in 1996. The unidentified author of the best-selling political novel “Primary Colors” was eventually exposed, too.

The Unabomber anonymously sent 16 mail bombs to various targets between 1978 and 1995, killing three people and injuring 23. Despite a massive FBI manhunt, the man later arrested and convicted of the crimes, Ted Kaczynski, was not identified until he wrote, and The Washington Post published, a lengthy and rambling environmental “manifesto” in 1995. Kaczynski’s brother recognized his writing style and tipped off authorities, who traced the former math prodigy to a shabby cabin in Montana seven months later.

Primary Colors” was a bestseller in 1996, largely because of an ingenious marketing gimmick: Its author was “Anonymous.” As a result, the roman à clef about Bill Clinton’s 1992 presidential campaign fueled a grand guessing game about who wrote it, hyping its sales. Time magazine columnist Joe Klein, then at Newsweek, repeatedly denied that he was the author, but eventually admitted it.

Klein said in an interview that his authorship was very closely held; only his family and his agent knew he had written the book. Even his editor, Dan Menaker, thought the author was a woman, a bit of mistaken identity that Klein said he considered “high praise.”

In the end, Klein said, he was done in by “a mistake”: He wrote a few words in the margins of a manuscript that somehow wound up in the hands of a Washington Post reporter, David Streitfeld (now at the New York Times). Streitfeld compared it with another sample of Klein’s handwriting and consulted an expert analyst. Game over.

Deep Throat was the legendary anonymous source for Post reporter Bob Woodward when he was reporting on President Richard Nixon in 1972 and 1973, who was made even more famous by Hal Holbrook’s portrayal of him as a shadowy underground parking-lot denizen in the movie version of Woodward and Carl Bernstein’s book, “All the President’s Men.”

Woodward never publicly revealed who Deep Throat was. The secret held despite extravagant third-party investigations (former Nixon aide Leonard Garment wrote an entire book about it in 2000, “In Search of Deep Throat,” fingering the wrong guy).

It wasn’t until 2005, as he neared death, that Mark Felt, a high-ranking FBI official during the Nixon years, stepped out of the metaphoric shadows and declared that he was Woodward’s source.

During all the decades of Deep Throat’s anonymity, Woodward said he revealed Felt’s name to only three people: Bernstein; his Post editor, the late Ben Bradlee; and his wife, Elsa Walsh.

“I always used to joke with Ben Bradlee that the only way three people could keep a secret is if two of them are dead,” said Woodward, citing a quip attributed to Benjamin Franklin. “There’s some truth to that, but people really are capable of keeping secrets if they want to.”

In an odd intertwining of history, Woodward played an indirect role in boosting the profile of the Times’s anonymous op-ed writer. During the week the article was published, Woodward had begun promoting his book about Trump, “Fear,” which documented infighting and chaos within his administration. The book and the op-ed told a consistent story, and the attention paid to one reinforced the other.

But Woodward is dismissive of the op-ed column now.

“What’s lacking in the op-ed piece are specifics,” he said. “If the person [who wrote it] had come to me when I was writing the book, I would have said, ‘What are the specifics? What did you see? What did you participate in?’

“If they couldn’t offer those details, I wouldn’t have put it in the book. I would have said, ‘Take it to the New York Times.’ ”

The author of the Times op-ed could not be reached for comment.