WASHINGTON — Robert S. Mueller III, the special counsel, declined on Wednesday to clear President Trump of obstruction of justice in his first public characterization of his two-year investigation of Russia’s interference in the 2016 presidential election.
“If we had had confidence that the president clearly did not commit a crime, we would have said so,” Mr. Mueller said, reading from prepared notes behind a lectern at the Justice Department at a hastily called appearance.
He also noted that while Justice Department policy prohibits charging a sitting president with a crime, the Constitution provides for another remedy to formally accuse a president of wrongdoing — a clear reference to the ability of Congress to conduct impeachment proceedings.
Although it lasted only 10 minutes, the news conference presented an extraordinary spectacle of a top law enforcement official publicly stating that the president’s conduct had warranted criminal investigation, even though it was impossible to indict him for any crimes.
Democrats seized on Mr. Mueller’s refusal to exonerate Mr. Trump and his reference to Congress, saying the onus was now on the House and Senate to fully investigate Mr. Trump’s actions. The president’s aides tried to cast the event as not even newsworthy, just a repetition of a 448-page report released weeks ago.
Mr. Mueller, 74, who carried out the investigation without publicly speaking a word, walked stiffly to the lectern and began tentatively, then delivered a succinct distillation of his investigation that was distinctly more powerful than the often legalistic language of his two-volume report.
In summing up his inquiry, he seemed to cast the president’s conduct in a more damning light than Attorney General William P. Barr. Mr. Barr has said that Mr. Trump was not guilty of “obstructive conduct.” He also suggested that the president may have been merely acting from frustration, not corrupt intent, when he tried to interfere with the special counsel’s investigation, including when he tried to oust Mr. Mueller.
Mr. Mueller, by contrast, stressed the gravity of the allegations against the president. Although he noted that his office did not “make a determination as to whether the president did commit a crime,” he said, “the matters we investigated were of paramount importance.”
He added, “When a subject of an investigation obstructs that investigation or lies to investigators, it strikes at the core of their government’s effort to find the truth and hold wrongdoers accountable.”
Mr. Barr suggested at one point that the special counsel should not have collected evidence against Mr. Trump if he intended only for Congress to review it. But Mr. Mueller defended his inquiry as wholly justified, even though he said charging the president with a crime was “not an option we could consider.”
The investigation was necessary both to preserve evidence and to hold accountable anyone who might have conspired with the president and could face criminal charges, he said. He also referred obliquely to the impeachment process, a point that is reduced to a footnote in his report.
“The Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing,” he said. He cited the same opinion from the Justice Department’s Office of Legal Counsel that states that a sitting president cannot be criminally charged.
He also said that the order by the former deputy attorney general, Rod J. Rosenstein, that appointed him specifically authorized him to investigate actions to interfere with his inquiry.
Mr. Mueller said his news conference was his final word on his investigation. He would not comment further, he said, on the actions of the Justice Department or Congress. He suggested that Democrats in Congress were wasting their time in seeking his testimony because he would simply repeat what he stated in his report. “The report is my testimony,” he said.
Mr. Trump’s aides, who were informed last night that Mr. Mueller intended to deliver a statement, pointed instead to Mr. Barr’s more favorable public statements about the president’s behavior. After Mr. Mueller declined to decide whether the president was guilty of a crime, Mr. Barr decided that evidence against the president was insufficient to warrant a criminal prosecution — a decision he reached with Mr. Rosenstein.
The White House press secretary, Sarah Huckabee Sanders, told reporters that further investigations by Congress were unnecessary because “it’s already been done.” She added, “We consider this case closed.”
Democrats said Mr. Mueller’s remarks only underscored the need for Congress to follow up with subpoenas and hearings.
“If he had any evidence that the president was not guilty, he would have let us know,” said Speaker Nancy Pelosi. “But he didn’t. He didn’t. And I think that was very, very important.”
She said Congress should continue to press for more information, including for passages in the report that were redacted to protect grand jury material, current criminal investigations or classified information.
Mr. Mueller began his statement by reviewing the highlights of how Russia had tried to influence the results of the 2016 election. He described how Russian operatives hacked into Democratic computers and released documents through WikiLeaks to damage the campaign of Hillary Clinton, whom he described only as a presidential candidate.
He returned to that theme at the end of his speech, emphasizing the gravity of Russia’s attack on the American political system.
“I will close by reiterating the central allegation of our indictments — that there were multiple, systematic efforts to interfere in our election,” he said. “And that allegation deserves the attention of every American.”
Although his comments suggested he was more troubled by Mr. Trump’s actions than Mr. Barr, Mr. Mueller took pains to defend how the attorney general handled his report — a subject of previous disagreement between the two men. In a brief letter in late March, Mr. Mueller had criticized Mr. Barr for sowing “public confusion” in his initial summary of the report, and had asked him to release his own executive summaries to clarify misimpressions.
Instead, Mr. Barr waited more than three weeks while the report was being redacted, then released it nearly in full in late April. The president’s allies took advantage of that interim to cast the findings in Mr. Trump’s favor, prompting complaints that Mr. Barr was allowing them to shape the public narrative before the facts came out.
Mr. Mueller acknowledged that he and Mr. Barr had differed over whether to release his team’s summaries. But he said, “I certainly do not question the attorney general’s good faith in that decision.” He also complimented Mr. Barr’s decision to make almost the entire report public.
While he made no reference to Mr. Trump’s repeated denunciations of his work as a “political hoax” carried out by “angry Democrats,” he described his investigation as “fair and independent.” He said the prosecutors and F.B.I. agents on his team were public servants “of the highest integrity” and thanked them for their work.
Despite Mr. Mueller’s comments, Democratic House leaders vowed to continue to seek his testimony.
“Given that the president has not been cleared of wrongdoing, and given the seriousness of Russia’s interference in our democracy, I believe that the American people deserve to hear testimony from the special counsel about his report and the report’s conclusions,” said Representative Steny H. Hoyer of Maryland, the No. 2 House Democrat.
Republicans, meanwhile, said they were looking ahead to the results of a review into the origins of the counterintelligence investigation that ultimately prompted Mr. Mueller’s appointment. Mr. Barr has assigned the review to the United States attorney in Connecticut but is personally overseeing it.
Department officials say he wants to understand on what basis the F.B.I. first began investigating ties between the Trump campaign and Russia in summer 2016.
Mr. Barr has also said he is concerned about the justification for “spying on Americans.” The F.B.I. sought a court order to conduct surveillance on a former Trump campaign aide, partly on the basis of research that was paid for by a law firm hired by Democrats.
Mr. Trump has praised the review, saying the investigation into him and his campaign was never justified.