The Justice Department's finding that there is insufficient evidence to charge President Donald Trump with obstruction of justice drew skepticism from congressional lawmakers Sunday, with one key Democrat vowing to summon U.S. Attorney General William Barr to explain the determination.

Rep. Jerrold Nadler, the top Democrat on the House Judiciary Committee, said his panel would call on Barr to testify before lawmakers “in the near future.” He pointed to the “very concerning discrepancies” behind the Justice Department's decision.

Nadler's reaction came after Barr, in a four-page letter sent to lawmakers, announced the decision to not pursue charges against Trump. Barr said he and Deputy Attorney General Rod Rosenstein made their determination after reviewing special counsel Robert Mueller III's findings, which were submitted to the attorney general Friday.

While Barr and Rosenstein determined there was insufficient evidence to establish the president obstructed justice, Mueller punted on the question, according to Barr's summary. Barr noted Mueller's findings set “out evidence on both sides of the question.”

“The special counsel states that, 'while this report does not conclude that the president committed a crime, it also does not exonerate him,'” Barr's summary said.

Lawmakers, including Nadler, seized on that apparent difference Sunday to signal they would haul in Barr, and potentially Mueller, for a possible congressional briefing.

The events unfolding Sunday evening offered a preview of the likely legal tussle between Congress and the Justice Department over the public release of Mueller's findings.

Legal experts said that Barr's report underscored the importance of releasing Mueller's findings. The U.S. House of Representatives voted 420-0 earlier this month in favor of publicly releasing the findings. But the resolution was nonbinding.

Neal Katyal, a former acting solicitor general who also drafted the special counsel rules in 1999, wrote in a New York Times column Sunday that Barr's letter raised more questions than it answered.

“Congress now has a clear path of action. It must first demand the release of the Mueller report, so that Americans can see the evidence for themselves. Then, it must call Mr. Barr and Mr. Mueller to testify,” said Katyal, now a partner at Hogan Lovells.

Ronald Klain, general counsel at Revolution LLC and a veteran of the Clinton White House, tweeted that Barr's decision on prosecution furthered the case for public release.

“Since the decision—by his own admission—not to prosecute on obstruction was AG Barr's (and NOT Mueller's), this makes the case for a full release of the Mueller Report even more compelling (so Congress can assess Barr's decision),” he said.

For his part, Trump on Sunday claimed “complete and total exoneration.”

In the attorney general's letter to Congress, Barr said he and Rosenstein—who until recently oversaw Mueller's inquiry—consulted Justice Department officials and lawyers, including the DOJ's Office of Legal Counsel and applied the “principles of federal prosecution that guide our charging decision.”

Lawmakers will likely seek answers on why Mueller never interviewed the president. Trump's personal attorneys had only agreed to send written responses to the special counsel's questions.

In his letter Sunday, Barr indicated he would withhold sensitive information related to grand jury proceedings from a subsequent report to Congress. But Barr also reiterated that he intends to release as much as he can under the law, regulations and DOJ policies. Barr added he has requested Mueller's assistance to identify all of that information “as quickly as possible.”

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