In the weeks after Donald Trump was elected president, CIA analysts who work on Russia updated their assessment about Russian cyber meddling. They previously had assessed that Russia was trying to undermine the U.S. presidential election, but after Nov. 8 they came to believe that Russian interference was ultimately designed to help Trump win.

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This latest finding fed an already blazing public dispute between the U.S. intelligence agencies and the president-elect over Russian interference in the election.

"I don't believe it," Trump said in an interview broadcast Sunday on Fox News, disputing the notion that the Russians wanted to help him. "I think it's ridiculous."

Trump and his aides reject not just the CIA’s updated assessment about Russian intentions, but the earlier consensus reached by all 17 agencies, and released in a rare joint public statement by the heads of the DHS and the Office of the Director of National Intelligence, that Russia was meddling in the election in the first place.

The seriousness of the gulf can hardly be overstated: many foreign policy experts and lawmakers believe Russia’s behavior was an attack on the nation. One former CIA leader, Hillary Clinton supporter Michael Morell, likened Russia’s intrusion into U.S. politics Friday to "the political equivalent of 9/11."

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In fact, the CIA’s latest take is not the unanimous view of the intelligence community. Director of National Intelligence James Clapper, who oversees all 17 U.S. intelligence agencies, had enough confidence to relay the CIA’s assessment to Congress in a secret briefing last week. But no similar assessment has leaked from any of the other 16 intelligence agencies.

The FBI, for its part, assesses that Russia’s main goal was to sow chaos and undermine American democracy, a senior law enforcement official told NBC News. Helping Trump served that purpose, the official said, but Russia didn’t expect Trump to win the election.

The CIA’s assessment that the Russians favored Trump was not based on any single piece of new intelligence, officials briefed on the matter told NBC News. Instead, it was the result of more stringent analysis of a growing body of circumstantial evidence more detailed than anything the public has seen.

Human sources, communications intercepts and other intelligence have allowed analysts to piece together the identities of some of the players, and the steps they took to hurt Clinton’s candidacy while boosting Trump’s, officials said.

The CIA also noted that while Russian hackers gathered information on Republicans, they didn't release any of it, the way they did with Democratic emails leaked to WikiLeaks. The Republican National Committee denies its systems were hacked, but the emails of individual Republicans were collected — something NBC News reported in October.

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The FBI believes the Russians didn’t find anything explosive in the Republican material they obtained, the official said, although it’s not clear whether any U.S. agency knows the full extent of what data Russian hackers were able to steal.

The CIA also tracked a campaign of anti-Clinton news stories amplified by social media, some of which originated with Russian state media outlets, officials told NBC News.

The case that the Russian campaign was explicitly intended to elect Trump is not something the U.S. could prove in court, officials say, which in part explains why FBI briefers have been more reluctant to embrace that position in secret briefings to Congress than have intelligence officials. Other intelligence agencies have not endorsed the CIA view, but no entity is known to oppose it.

However, the question of motive — why the Russians allegedly meddled in the election — is entirely separate from the more basic question of whether the U.S. has strong evidence that the Russian government sponsored the covert campaign.

On that score, every intelligence agency in the government agrees: The Russians did it.

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In October, DNI Clapper and Homeland Security Secretary Jeh Johnson issued an extraordinary joint statement that "the U.S. Intelligence Community is confident that the Russian Government directed the recent compromises of e-mails from US persons and institutions, including from U.S. political organizations."

One source familiar with the intelligence said it included profiles of some of the suspects with pictures, and could lead to eventual indictments of specific Russian actors, if the Justice Department decides to pursue them.

The evidence on that score simply doesn’t support Trump’s recent comments that "It could be Russia. And it could be China. And it could be some guy in his home in New Jersey."

That is why the two Trump supporters who lead the Congressional intelligence committees, Sen. Richard Burr of North Carolina and Rep. Devin Nunes of California, have each issued statements in recent days expressing concern about Russian hacking.

"The Senate Select Committee on Intelligence has been, and remains, concerned about Russia's actions," Burr said in a statement to NBC News Friday.

Nunes said Friday that "Russia’s cyber attacks are no surprise to the House Intelligence Committee, which has been closely monitoring Russia’s belligerence for years."

On Monday, the top Republicans in Congress — Majority Leader Mitch McConnell and Speaker Paul Ryan — each issued statements pledging to investigate Russian meddling in the election.

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Ryan called Russia "an aggressor," and McConnell made a point of saying he has "the highest confidence in the intelligence community, and especially the Central Intelligence Agency."

On Monday, Sen. John McCain of Arizona renewed his call for a bipartisan hearing, but said he didn’t believe Russians were trying to engineer a Trump victory: "That’s the reason we need a full investigation, but I have seen no evidence of that."

It’s unclear whether Trump has been briefed on the same evidence top Republicans have seen, because he has had only a handful of intelligence briefings since the election.

Trump not only said he didn't believe the intelligence community's assessment — he suggested that the CIA was not to be believed in general, issuing a statement Friday night noting that "These are the same people that said Saddam Hussein had weapons of mass destruction."

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Several intelligence officials told NBC News they were deeply disturbed by that statement. There is no historical precedent for a president-elect publicly maligning the intelligence agencies he is about to lead.

One big question now is how Trump’s pick as CIA director, Mike Pompeo, will bridge the acrimonious gap between his boss and the agency he leads.

President Obama does not intend to make it easy on Trump: He has ordered that a dossier be assembled on the evidence about the Russian covert operation.

A senior Obama administration official told NBC News that portions of that will be made public before January 20.

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Sen. Angus King, a Maine Independent who serves on the Senate Intelligence Committee, told NBC News that can’t happen soon enough. The way to thwart Russian information campaigns is to publicize them, he said.

He warned, though, that the U.S. Is not going to be able offer the sort of proof many skeptics are demanding.

"If you provide proof, you provide a roadmap to show the Russians how we caught them," he said, "including intelligence sources and methods."