Author Kurt Eichenwald (Jessica D’Onofrio)

Kurt Eichenwald is an experienced investigative journalist, with a best-selling book to his credit and years of work at the New York Times, Vanity Fair and now Newsweek.

So what was Eichenwald thinking Tuesday when he tweeted this:

“I believe Trump was institutionalized in a mental hospital for a nervous breakdown in 1990, which is why he won’t release medical records.”

There’s no evidence that Trump was ever hospitalized for such a condition, and Eichenwald cited none.

The tweet was quickly deleted from Eichenwald’s account, though not before it received hundreds of retweets and “likes,” spreading it far and wide across the Twittersphere.

Eichenwald and his Newsweek editor, Jim Impoco, didn’t respond to repeated requests for comment Wednesday. But his bizarre and apparently erroneous tweet may offer another lesson in how social media has made it tempting for reporters to overstep the boundaries of professional conduct.

The ease of tweeting or posting on Facebook has sorely tempted more than a few journalists to do what they’re not supposed to do in their day jobs — namely, float opinions or publish rumors that would never meet the standards of their publications or networks.

While news organizations encourage journalists to engage with readers via social media, some reporters have gone further than merely fostering discussion or promoting their work. Reporters have lost their jobs or been disciplined by their employers for posts that call into question the reporter’s fairness or neutrality on a story. CNN’s Elise Labott, for example, was suspended for two weeks last fall for tweeting a partisan comment about a House vote on refu­gees.

Most news organizations have elaborate codes of conduct for what’s acceptable online. The rules basically boil down to this: Don’t post anything that wouldn’t be acceptable in a news story.

“In our view, journalism is journalism no matter where it occurs,” said Andrew Seaman, the chairman of the Society of Professional Journalists’ ethics committee. “So if it’s not good enough for the paper, it isn’t good enough for social media. . . . The quality of the journalism should remain the same.”

Seaman, a reporter for Reuters, had a succinct reaction on Twitter to Eichenwald’s deleted tweet Tuesday: “Completely irresponsible.”

Eichenwald’s tweet appears to cross the usual lines. It is an unsupported and potentially inflammatory claim about a leading presidential candidate in the midst of a campaign.

Trump has sued reporters for controversial, though more carefully reported, statements before; he brought (and lost) a libel suit against then-New York Times reporter Timothy L. O’Brien for O’Brien’s contention in a 2005 book, “Trump Nation,” that Trump’s net worth was substantially less than Trump claimed.

More recently, Trump’s wife, Melania, has objected to published but unsubstantiated claims that she was employed by an escort service in the 1990s. She has hired Charles Harder, the lawyer who spearheaded the invasion-of-privacy suit brought by former wrestler Hulk Hogan against Gawker Media, to seek redress.

Eichenwald’s Trump tweet came the day before Newsweek published a much-discussed story by him about the Trump Organization, the candidate’s sprawling business empire. The story raised important questions about potential conflicts of interest between Trump’s extensive business holdings and U.S. national-security interests if he is elected president.

Eichenwald teased the story by tweeting on Tuesday afternoon, “My big cover story in @Newsweek that could change the dialogue about this election season will be published online tomorrow.”

A few minutes later, he posted his now-deleted comment about Trump’s mental health, sowing confusion about what his Newsweek story would be about.

Not long after that, he tweeted that the two topics were unrelated: “Folks: I don’t have story coming out tomorrow that Trump had a breakdown. Order of tweets confused ppl, I believe.”

He later said he deleted the tweet about Trump’s mental health “because people were confusing it” with his forthcoming Newsweek story.

But Eichenwald never said he did so because it was wrong.

Seaman, in an interview, said journalists should hold themselves to a higher standard. “I think it’s a heavy claim for a prominent journalist to just put [speculation] out on social media without additional evidence,” he said. “Speculation isn’t journalism, and I think it makes the public distrust solid and evidence-backed reporting.”