(CNN) Corey Lewandowski had a blunt message for Democrats on the House Intelligence Committee: He wasn't going to answer their "fucking" questions.

Lewandowski, President Donald Trump's former campaign manager , was the final witness in the yearlong House investigation that descended into vitriol and back-biting -- ultimately resulting in two separate partisan reports that will leave the American public no closer to learning how the Russians interfered in the 2016 elections.

But Lewandowski, who agreed to come back to the committee a second time in March after initially refusing to answer questions about topics occurring once he left the campaign in June 2016, was in no mood to give Democrats anything they wanted, saying he would only answer "relevant" questions.

And, according to four sources with direct knowledge of the situation, the Trump confidante repeatedly swore at Democratic lawmakers to make the point he wasn't going to talk further.

I'm not answering your "fucking" question, Lewandowski shouted at one point.

Democrats, including Rep. Jackie Speier, fired back at Lewandowski, who was not moved, multiple sources said.

Democrats said that Lewandowski wouldn't discuss the firing of FBI Director James Comey, the White House response to revelations that Donald Trump Jr. met with Russians in Trump Tower in June 2016 and his conversations with the President, among other topics.

Republicans sided with Lewandowski, saying he had spent hours before the panel answering questions pertinent to the inquiry.

At left, Rep. Jackie Speier, a California Democrat, and, at right, former Trump campaign manager Corey Lewandowski.

And in an interview this week, Lewandowski did not dispute using those words.

"I had to repeat on multiple occasions that there was no collusion, cooperation or coordination because the Democrats couldn't understand my plain English way of speaking," Lewandowski told CNN.

After this story published, Lewandowski told CNN that Democrats on the committee were the first to use foul language during his testimony, saying, "I felt the need to respond in kind." He said he did not shout at the members.

"The language they used in the committee was appalling," Lewandowski said. "I've never heard such language used before."

Lewandowski said he did "not recall" precisely what was said, and declined to name the Democratic members of the committee he was referring to.

The previously unreported episode underscores just how far the House investigation had fallen off the rails, marking a fitting end to an investigation that began with a bipartisan announcement and news conference from House Intelligence Chairman Devin Nunes and Rep. Adam Schiff, the top Democrat — and quickly devolved into partisan acrimony.

"There wasn't one tipping point that changed the mood of the committee," said Rep. Chris Stewart, a Utah Republican who sits on the panel. "It wasn't a slash with a sword, it was more of 1,000 paper cuts that wore away and eroded that."

This account of how the House Intelligence Committee investigation broke down is based on more than a dozen interviews with committee members, current and former aides and others who interacted with the committee's investigation. Some officials spoke on the record and others insisted on anonymity to discuss sensitive committee matters.

Democrats cast the blame primarily on Nunes, arguing that he was out to undermine the probe from the get-go, a member of the Trump transition team whose primary goal was to protect the President. Egged on by Nunes' staff, the Democrats say, Rep. Mike Conaway and other Republicans willingly went along with an effort to rush the probe to conclude there was no collusion.

Schiff said Conaway, the Republican in charge of the Russia investigation after Nunes stepped aside last April, was "forced on the most important decisions to confer" with Nunes, who sat on numerous Democratic requests for subpoenas and refused to schedule dozens of witness interviews.

"I can only conclude that he views his mission as protecting the President," Schiff said of Nunes.

But Republicans argue that Schiff and his fellow Democrats shoulder the blame for the committee's partisan war, starting with the push to remove Nunes as head of the committee with what they argue were trumped-up, partisan ethics charges. They say that Democrats have tried to draw out the investigation as long as possible once the anticipated smoking gun proving collusion failed to materialize.

"Given the overall backdrop of how many folks don't like Trump, how many folks do like Trump — and the raw partisan nature of all of that and that debate — I don't know that we could have done it differently or it would have wound up any differently," Conaway told CNN.

While Republicans and Democrats are at odds about nearly everything involving the investigation, there is one thing they now agree on: The writing had been on the wall for months — if not from the beginning — that the probe into Trump and Russia was doomed to devolve into a partisan brawl.

Regardless of the narrative, lawmakers on both sides point to a fateful week in March 2017 as the key turning point from which the House Intelligence Russia investigation was never able to recover.

A trip to the White House

One day after then-FBI Director James Comey told the committee in March 2017 that the FBI was investigating ties between Trump and Russia, Nunes took a clandestine trip to the White House grounds, claiming to be in possession of information showing Trump officials' communications were intercepted by US intelligence. The next day, Nunes held a news conference and went to the White House to brief the President, a move that set off a firestorm from Democrats, prompting Schiff to call for Nunes to recuse himself from the Russia investigation.

Nunes initially refused, but the pressure on him mounted as more details emerged about how he had received the information from two officials at the White House. Just over two weeks later, amid a new House Ethics Committee probe into whether he had revealed classified information while discussing the intercepts, Nunes stepped aside and put Conaway in charge, along with by Reps. Tom Rooney of Florida and Trey Gowdy of South Carolina.

For a time, there was a "burst of cooperation" between Conaway and Schiff after the Texas Republican took over, according to sources from both parties.

But the scar tissue on the committee over the Nunes episode hadn't disappeared, and the episode left both sides deeply untrusting of the other.

Some Republicans believed committee Democrats were complicit in the use of an underhanded tactic to force Nunes aside, after liberal advocacy groups filed the complaint that prompted the Ethics Committee investigation, which ultimately cleared Nunes in December.

"The fact that an ethics complaint was filed against him was crossing a line," said a Republican close to the committee. "It was irreconcilable as to how the parties would work together where one views they're on a political revenge march versus an investigation. ... Not one Democrat on the committee willing to say this was destructive, this was baseless."

But to Democrats, Nunes' secret trip to the White House had only proven their worst suspicions that point of the investigation was to clear the President.

"It's hard to argue he hadn't pre-planned this would make some effort to show that they were trying to find out what happened, but to make sure it was guarded at all times," said Rep. Mike Quigley, an Illinois Democrat. "This was a designed failure. It was designed to tank, and most importantly protect the President politically and legally. And it succeeded in their efforts."

Nunes steps aside, sort of

While Nunes publicly had stepped aside from the probe in April 2017, he remained involved in quiet ways.

Democrats were surprised to learn Nunes retained subpoena power for the Russia investigation, and they say Nunes frequently blocked their efforts to issue subpoenas or call witnesses, although Conaway says he received any subpoena he has asked for from Nunes.

And Nunes had subpoenas of his own, too.

When the committee issued its first Russia-related subpoenas, for instance, to former national security adviser Michael Flynn and Trump's personal lawyer Michael Cohen, Nunes at the same time also subpoenaed former Obama administration officials over "unmasking," or revealing the identities of Americans who were communicating with foreign officials under surveillance by the US intelligence community.

Subpoenas would continue to be a sore spot in the investigation, particularly when Lewandowski, former White House chief strategist Steve Bannon and former White House communications director Hope Hicks testified earlier this year and would not discuss events after Trump was in the White House. Last month, Schiff released a list of nearly three dozen people and entities he thought should have been subpoenaed in the investigation.

Nunes would continue to loom over the Russia investigation in other ways.

Two of his aides took a trip in the summer to London in an attempt to find Christopher Steele, the former British spy who compiled a dossier of allegations of Trump ties to Russia, without alerting Democrats or even Conaway, although committee officials have said the staffers were only looking to contact Steele's attorney, not Steele himself.

Conaway has downplayed the trip's significance, saying the staffers were already in London on other committee business. But one committee source told CNN that the trip occurred as there were early discussions underway about having Conaway and Schiff try to go speak to Steele together.

Nunes was also working on his own investigation in parallel to the Russia probe, an effort focused on the Steele dossier. His committee went to court to enforce a subpoena obtaining the bank records of Fusion GPS, the firm that paid Steele, and he threatened to hold senior Justice Department and FBI officials in contempt over documents related to the Foreign Intelligence Surveillance Act surveillance warrant on former Trump campaign adviser Carter Page.

Nunes' efforts culminated in the memo he released in January accusing the FBI of abusing the surveillance process with the warrant on Page. The Nunes memo sparked a counter-memo from Democrats and threw the committee into a month-long feud that was arguably its most-heated partisan fight.

A spokesman for Nunes declined to comment.

Fights in public and private

The reality was the partisan brawling sparked by the memos was a product of months of tension bubbling under the surface.

During last summer as the committee investigation marched on, Schiff and Conaway held weekly meetings about the investigation. But as summer turned to fall, the meetings became less frequent and then stopped happening at all, according to a committee source.

At the same time, the pace of the witness interviews accelerated, often with multiple interviews in a single day in November and December. Witnesses, at times, would meet with the panel before members had a chance to review any records that were submitted to the committee, which Democrats said impaired their ability to ask relevant questions.

The quickened schedule prompted Democratic predictions that Republicans were trying to shut down the investigation soon after the calendar turned to 2018, while Republicans began accusing Democrats of trying to stretch out the investigation into the midterm campaign season.

Many Republicans on the committee grew agitated with the frequent television appearances of Schiff and other committee Democrats. Several Republicans suggested Schiff was using the committee to position himself for a potential Senate run had Sen. Dianne Feinstein retired (she has since said she'll run for re-election).

Schiff disputed that contention, pointing to the frequent television appearances by Gowdy.

"What bothered them was that we were exposing their malfeasance," Schiff said of Republicans.

Republicans also accused their Democratic counterparts of leaking committee testimony and documents to drive anti-Trump stories. The committee's Republicans even tallied the list of leaks they said had originated from Democrats, a statistic that was included in materials the committee and Speaker Paul Ryan's office used to defend the investigation.

But the leak accusations came from both sides: Donald Trump Jr.'s attorney alleged that Democrats were leaking testimony in the middle of his client's hearing, while Nunes and committee Republicans were accused of leaking witness testimony to Cohen's attorney, as well as the text messages of Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee.

The partisan accusations didn't stop when the committee interviewed witnesses in closed settings either.

One Republican close to the panel said that during witness interviews, "Schiff and his team could not stop attacking their Republican counterparts."

Democrats, meanwhile, said Republicans asked merely superficial questions — simply asking Trump officials if they had colluded cooperated or coordinated with Russians — and tried to protect witnesses like Trump's son-in-law, Jared Kushner.

And now Democrats are pushing for the release of the transcripts, something Conaway once said he was open to but has since backed away from.

"They are now doing a complete about-face and have flip-flopped on this," Schiff said.

Is this the end?

Last month, the committee's Republicans announced they had concluded their investigation and had a 150-page report with the panel's findings, a number that ballooned by roughly 100 pages by the time the committee voted out the report the following week, according to Conaway.

The report, which hasn't been made public yet, found no evidence of collusion, a conclusion that Republicans have stood by but Democrats say was engineered from the start.

"We set out with four goals that we hope to accomplish with the committee, and by and large we accomplished those goals," Stewart said. "The committee has had a breakdown in bipartisanship, but I don't think that entirely stopped us from doing the work we need to do."

But the Russia fight doesn't appear over. Democrats are preparing a separate report from Republicans, and they're also starting their own Russia investigation with witnesses willing to come forward, like former Cambridge Analytica contractor Christopher Wylie.

And Nunes' investigation into the dossier is continuing, too, with a probe of the State Department, as he tries to determine who else in the Obama administration was aware of the Steele dossier.

"The Republicans haven't stopped the investigation," Schiff said. "It's just the Russia piece they don't want to do anymore."

This story has been updated to include additional comment from former Trump campaign manager Corey Lewandowski.