Facing calls from Republicans to step down over his "collusion" claims, House Intelligence Committee Chairman Adam Schiff on Thursday maintained there is still much to learn following the end of Robert Mueller’s Russia investigation—shifting focus to the original FBI counterintelligence probe that started it all.

A summary of Mueller’s report was released by Attorney General Bill Barr over the weekend and announced that after an exhaustive investigation that lasted nearly two years, the special counsel found no evidence of collusion between members of the Trump campaign and the Russians during the 2016 presidential campaign.

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But while some version of Mueller’s report will likely be released in the near future, Schiff, D-Calif., suggested during a committee hearing on Thursday that the most important pieces of information would come from the original counterintelligence investigation launched under former FBI Director James Comey.

“We do not yet know the result of the counterintelligence investigation led by Comey and then Mueller,” Schiff said in his opening statement. “It is not clear whether, or to what extent, the Mueller report, which is focused on prosecutorial decisions, will even discuss counterintelligence findings.”

He added: “But the counterintelligence concerns that animated the investigation in the first place may ultimately have the more profound impact on our security and policy, and we therefore await the publication of the Mueller report to find out.”

The comments come as all Republicans on the committee called on Schiff Thursday to step down as chairman, over his repeated claims to have evidence of collusion. “Your actions both past and present are incompatible with your duty as Chairman of this Committee,” their letter to Schiff stated. “We have no faith in your ability to discharge your duties in a manner consistent with your Constitutional responsibility and urge your immediate resignation as Chairman of this Committee.”

A day earlier, Schiff told The Daily Beast that he has started negotiations with the intelligence community to get answers about the Mueller investigation’s findings, and reportedly signaled that the FBI’s counterintelligence probe that evolved into the Mueller investigation could still be ongoing.

“At this point, we don’t know whether any of the counterintelligence findings are part of the Mueller report,” Schiff told The Daily Beast. “We have initiated discussions with the intelligence community to make sure that we obtain whatever is found in the counterintelligence investigation, or whether [that inquiry] is still ongoing.”

The FBI declined to comment to Fox News when asked if the initial counterintelligence probe led by the bureau was ongoing or complete.

The FBI’s initial counterintelligence probe began in the summer of 2016 and was internally called “Crossfire Hurricane.”

Former FBI counsel Lisa Page said during a transcribed interview with the House Judiciary Committee last year that the FBI only had a “paucity” of evidence at the start of the investigation. Page also said the FBI “knew so little” about whether allegations against the Trump campaign were “true or not true” at that time.

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President Trump, this week, vowed to release the full and unredacted Foreign Intelligence Surveillance Act (FISA) warrants and related documents used by the FBI to probe his campaign and former campaign aide Carter Page.

Redacted versions of the FISA documents already released have revealed the FBI extensively relied on documents produced by ex-British spy Christopher Steele, who worked for Fusion GPS—a firm hired by the Hillary Clinton campaign and the Democratic National Committee to compile opposition research on Trump. The FBI relied on much of Steele’s work to obtain the warrants to surveil Carter Page.

“I have plans to declassify and release. I have plans to absolutely release," Trump said during an exclusive interview on “Hannity” Tuesday night. "I have some very talented people working for me, lawyers, and they really didn't want me to do it early on. ... A lot of people wanted me to do it a long time ago. I'm glad I didn't do it. We got a great result without having to do it, but we will. One of the reasons that my lawyers didn't want me to do it, is they said, if I do it, they'll call it a form of obstruction."

Trump added: "At the right time, we will be absolutely releasing."

Fox News' Gregg Re contributed to this report.