House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer waited more than two hours after the letter's release to issue a statement, writing that it "raises as many questions as it answers" and continuing to call for the report's full release.
The Democratic leaders echoed Nadler in pointing out that Barr's letter notes that Mueller did not exonerate Trump on the question of obstruction.
They wrote it "demonstrates how urgent it is that the full report and underlying documentation be made public without any further delay. Given Mr. Barr's public record of bias against the Special Counsel's inquiry, he is not a neutral observer and is not in a position to make objective determinations about the report."
Details of the Investigation
Barr wrote that Mueller interviewed about 500 witnesses, made requests to 13 foreign governments for evidence and obtained more than 230 orders for communications records.
He also said that Mueller hasn't recommended indictments against anyone else and that there are no sealed indictments that have yet to be made public.
The news about no more charges was part of the announcement on Friday when Mueller notified the leaders of the Justice Department that he had completed his work, which began in May 2017.
For much of the weekend, Washington has been anticipating the conclusions from Barr, who spent Saturday and Sunday in his office, along with Rosenstein.
According to a senior Justice Department official, Barr's chief of staff called White House lawyer Emmet Flood at 3 p.m. to give him a "readout" of the letter.
Mueller has not been at the Justice Department this weekend, and he "was not consulted on this letter," according to the department.
The Justice Department is not giving a timeline as to when it may release additional material from the Mueller investigation, and it will review the report and materials for grand jury information and for information that could implicate ongoing investigations.
Widespread Calls for Openness
Calls have been nearly universal and bipartisan for Mueller's original report to be released and for the public or Congress to access its findings and its underlying source material.
Many Republicans and Democrats agree that they will not be content only with a Barr-drafted synopsis of the report.
"It needs to be released to the Congress, and it needs to be released to the American people," Sen. Ted Cruz, R-Texas, told CNN on Sunday morning. "This has consumed two years of the American people's time, and we need full transparency."
Congressional Democrats appeared nearly unanimous about the need for wide release, and individual Democratic leaders also tried to pre-empt what they feared might be attempts by the White House to conceal Mueller's findings.
Executive Privilege
Specifically, one long-standing question has been whether Trump might seek to invoke executive privilege, the doctrine that allows an administration to keep secret some of its internal workings.
Nadler, the House Judiciary Committee chairman, told NBC on Sunday morning that he thinks the president shouldn't attempt it.
"I do not believe it exists here at all because, as we learned from the [Richard] Nixon tapes case, executive privilege cannot be used to hide wrongdoing," Nadler said.
Continued Nadler: "In that case, the Supreme Court, nine to nothing, ordered that all the claims of executive privilege be overridden and the tapes be public. ... The president may try to assert it, may try to hide things behind it. But I don't think that's right or [will] be successful."
Trump Hasn't Opposed Publication
Trump, for his part, has said in the past both that he doesn't mind if Mueller's report becomes public — because he says he has done nothing wrong — and that there shouldn't have been a Mueller report in the first place.
Trump has gone back and forth about what he accepts about Russian interference in 2016, but he has been consistent that neither he nor anyone in his campaign had any connection to it.
That idea is a "hoax" perpetuated by conspirators and Democrats who are sore that Clinton lost to him, Trump says, and who have been consequently running a "witch hunt" against him.
NPR reporters Susan Davis, Tamara Keith, Ayesha Rascoe and Kelsey Snell contributed to this report.