Democrats showed no signs of letting up even after Special Counsel Robert Mueller’s report found no evidence of collusion between Trump campaign associates and Russia during the 2016 election, with their congressional leaders calling for not only the release of the full report but also the underlying documents -- and testimony from Attorney General Bill Barr.

Barr released a four-page summary Sunday of Mueller’s findings from his investigation that spanned nearly two years. Barr’s letter noted that while Mueller found no evidence of collusion, the special counsel did not come to a conclusion on whether President Trump obstructed justice.

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In turn, Barr and Deputy Attorney General Rod Rosenstein, who appointed Mueller and oversaw the majority of the Russia investigation after former Attorney General Jeff Sessions recused himself, concluded that the evidence was “not sufficient to establish that the President committed an obstruction-of-justice offense.”

This raised red flags for Democrats who already have signaled that the Mueller report is by no means the end of Trump investigations. If anything, Democrats were emboldened to keep pursuing the obstruction issue after Mueller left it open -- no matter what Barr concluded. Democrats quickly challenged Barr's impartiality while calling for a sweeping amount of documentation from the Mueller investigation itself.

“Attorney General Barr’s letter raises as many questions as it answers. The fact that Special Counsel Mueller’s report does not exonerate the president on a charge as serious as obstruction of justice demonstrates how urgent it is that the full report and underlying documentation be made public without any further delay,” House Speaker Nancy Pelosi, D-Calif., and Senate Minority Leader Chuck Schumer, D-N.Y., said in a joint statement Sunday night, blasting Barr as “not a neutral observer” and claiming he “is not in a position to make objective determinations about the report.”

“Congress requires the full report and the underlying documents so that the Committees can proceed with their independent work, including oversight and legislating to address any issues the Mueller report may raise,” they continued. “The American people have a right to know.”

Fox News has learned that Mueller told Barr and Rosenstein three weeks ago that he would not be able to reach a conclusion on obstruction of justice. One source described this as unexpected news.

With Mueller’s decision not to issue a conclusion on the obstruction of justice issue, Democrats on the House Judiciary Committee said that it “[s]eems like the Department of Justice is putting matters squarely in Congress’ court.”

House Judiciary Committee Chairman Jerrold Nadler, D-N.Y., vowed to have Barr testify before the committee “in the near future,” and said there “must be full transparency” on what Mueller uncovered “to not exonerate the President from wrongdoing.”

“DOJ owes the public more than just a brief synopsis and decision not to go any further in their work,” he tweeted, adding that since Mueller did not exonerate Trump on obstruction of justice, “we must hear from AG Barr about his decision making and see all the underlying evidence for the American people to know all the facts.”

Later, Nadler, House Intelligence Chairman Adam Schiff, D-Calif., and House Oversight Committee Chairman Elijah Cummings, D-Md., issued a joint statement calling into question Barr’s “objectivity” and calling "for the release of the Special Counsel’s full and complete report and all underlying documents."

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But Barr’s letter made clear that Mueller's probe was comprehensive, raising questions as to what Democrats might hope to uncover on their own -- especially without the prosecutorial muscle and leverage employed by Mueller. Barr’s letter outlined that the special counsel “issued more than 2,800 subpoenas, executed nearly 500 search warrants, obtained more than 230 orders for communications records, issued almost 50 orders authorizing use of pen registers, made 13 requests to foreign governments for evidence, and interviewed approximately 500 witnesses.”

Barr also stated that “in cataloguing the President’s actions, many of which took place in public view, the report identifies no actions that, in our judgment, constitute obstructive conduct, had a nexus to a pending or contemplated proceeding, and were done with corrupt intent, each of which … would need to be proven beyond a reasonable doubt to establish an obstruction-of-justice offense.”

Nevertheless, the comments from Democrats underscore that they have no intention of accepting Barr’s summary report as-is—amid numerous calls for the full report’s release—or putting the collusion and obstruction issues to rest, as they continue to pursue their own congressional investigations.

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Nadler’s committee is already leading an investigation into “alleged obstruction of justice, public corruption, and other abuses of power by President Trump,” which he announced earlier this month. Nadler requested documents and records from 81 individuals and entities connected, in some way, to the president.

Meanwhile, other House Democrats have aggressively ramped up their own Trump-related investigations, which are likely to include a network of committee probes and high-profile hearings that are likely to last well into the 2020 election year.

At this point, the House Intelligence Committee is probing the president’s foreign business dealings and Russian election meddling, and the House Foreign Affairs Committee, led by Chairman Eliot Engel, D-N.Y., is also probing the president’s foreign business dealings.

Fox News' Jake Gibson contributed to this report.