MSNBC's Lawrence O'Donnell on Wednesday retracted his story about supposed Russian ties to President Trump's finances and apologized for reporting it — just as Mr. Trump's lawyer demanded. O'Donnell said, however, he still doesn't know whether or not the story is true.

His public pull-back unfolded quickly in the opening minute of "The Last Word," where 24 hours earlier the cable news host said a source had told him that Deutsche Bank documents showed that Russian oligarchs had co-signed a loan application for Mr. Trump.

O'Donnell reported the story, based on a single source he did not identify, even as he couched it with the qualifier "if true" and admitted it had not been verified by NBC News.

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Mr. Trump's lawyer, Charles Harder, had written to NBC Wednesday afternoon, saying the story was false and defamatory and threatening legal action if it wasn't disowned. Harder said the story could have been disproven with an internet search.

O'Donnell quickly tweeted that he made an "error in judgment" reporting the story.

"We don't know whether the information is inaccurate," he said later on the air. "But the fact is we do know it wasn't ready for broadcast and for that I apologize."

He later apologized on air, saying the story "wasn't ready for broadcast." NBC News has not said whether or not O'Donnell faces disciplinary action.

In court Tuesday, Deutsche Bank had revealed that it possessed some of Mr. Trump's tax returns that had never been released to the public, leading to speculation about what information those documents contained.

When he initially reported on the supposed co-signing of the loans on Tuesday, O'Donnell said "that would explain, it seems to me, every kind word that Donald Trump has ever said about Russia and Vladimir Putin, if true, and I stress the 'if true' part of this."

The episode exhibited a stunning lack of rigor for a news organization. In a letter to Susan Weiner, NBC Universal's general counsel, and Daniel Kummer, the company's senior vice president for litigation, Harder called O'Donnell's statements "false and defamatory, and extremely damaging."

Harder said Mr. Trump was the only guarantor of the loans in question.

The erroneous reporting only helped provide ammunition for Mr. Trump's frequent attacks on the media. Early Thursday the president dubbed the host "crazy Lawrence O'Donnell" in a series of tweets, calling O'Donnell's reporting "the most ridiculous claim of all."

"Totally false, as is virtually everything else he, and much of the rest of the LameStream Media, has said about me for years. ALL APOLOGIZE!" Mr. Trump added.

The president's two eldest sons also took to social media to lambast the TV host.

This was a reckless attempt to slander our family and smear a great company. Apologies are not enough when the true intent was solely to damage and cause harm. As a company, we will be taking legal action. This unethical behavior has to stop. @Lawrence @MSNBC https://t.co/i4X6CcdkCm — Eric Trump (@EricTrump) August 29, 2019

"This was a reckless attempt to slander our family and smear a great company. Apologies are not enough when the true intent was solely to damage and cause harm. As a company, we will be taking legal action. This unethical behavior has to stop," tweeted Mr. Trump's son Eric.

His brother, Donald Trump Jr., echoed his calls, saying O'Donnell had "totally fabricated" his reporting.

The episode is a blow to MSNBC, which has built powerful ratings with a prime-time lineup of liberal commentators Chris Hayes, Rachel Maddow and O'Donnell and is a comfortable home to Trump opponents. Many nights, it is the second most-watched cable network, behind only Fox News Channel, whose lineup of conservative hosts appeals to the president's supporters.

But it also shows the inherent tension in the business model of building programming on news networks that are not necessarily run by journalists.

O'Donnell has been hosting "The Last Word" since 2010 and has been an MSNBC analyst since 1996. The Harvard College graduate was an aide to the late New York Sen. Daniel Patrick Moynihan and an executive producer on the NBC entertainment series "The West Wing."